Catalyzes a base-exchange reaction in which the polar head group of phosphatidylethanolamine (PE) or phosphatidylcholine (PC) is replaced by L-serine. a CDP-1,2-diacyl-sn-glycerol + L-serine = a 1,2-diacyl-sn-glycero-3-phospho-L-serine + CMP + H(+) Phospholipid metabolism; phosphatidylethanolamine biosynthesis; phosphatidylethanolamine from CDP-diacylglycerol: step 1/2. Belongs to the CDP-alcohol phosphatidyltransferase class-I family.