Small subunit of the glutamine-dependent carbamoyl phosphate synthetase (CPSase). CPSase catalyzes the formation of carbamoyl phosphate from the ammonia moiety of glutamine, carbonate, and phosphate donated by ATP, constituting the first step of 2 biosynthetic pathways, one leading to arginine and/or urea and the other to pyrimidine nucleotides. The small subunit (glutamine amidotransferase) binds and cleaves glutamine to supply the large subunit with the substrate ammonia