Catalyzes the key rate-limiting step in the CDP-choline pathway for phosphatidylcholine biosynthesis. CTP + H(+) + phosphocholine = CDP-choline + diphosphate By phosphorylation. Activated by N-methylethanolamine (PubMed:8185307). Activated by oleic acid-containing phosphatidylcholine vesicles (PubMed:8381041). Phospholipid metabolism; phosphatidylcholine biosynthesis; phosphatidylcholine from phosphocholine: step 1/2. Homodimer. It can interconvert between an inactive cytosolic form and an active membrane-bound form. The serine residues of the C-terminus are phosphorylated. The inactive soluble form is stabilized by phosphorylation, the active membrane bound form is promoted by anionic lipids or diacylglycerol, and is stabilized by dephosphorylation. The N-terminus is blocked. Monoubiquitinated by the SCF(FBXL2) complex, leading to proteasomal degradation. The cytidylyltransferase may interact with membranes through its amphipathic helix. Belongs to the cytidylyltransferase family.