Catalyzes the second two steps of the methylation pathway of phosphatidylcholine biosynthesis, the SAM-dependent methylation of phosphatidylmonomethylethanolamine (PMME) to phosphatidyldimethylethanolamine (PDME) and of PDME to phosphatidylcholine (PC). Can also catalyze the first methylation reaction of PE to PMME in the absence of PE methyltransferase CHO2. a 1,2-diacyl-sn-glycero-3-phosphoethanolamine + S-adenosyl-L-methionine = 1,2-diacyl-sn-glycero-3-phospho-N-methylethanolamine + H(+) + S-adenosyl-L-homocysteine 1,2-diacyl-sn-glycero-3-phospho-N-methylethanolamine + S-adenosyl-L-methionine = 1,2-diacyl-sn-glycero-3-phospho-N,N-dimethylethanolamine + H(+) + S-adenosyl-L-homocysteine 1,2-diacyl-sn-glycero-3-phospho-N,N-dimethylethanolamine + S-adenosyl-L-methionine = a 1,2-diacyl-sn-glycero-3-phosphocholine + H(+) + S-adenosyl-L-homocysteine Optimum pH is 8.1. Phospholipid metabolism; phosphatidylcholine biosynthesis. Expression is repressed by inositol and choline. The 5' flanking region contains two copies of the CATRTGAA motif and a 5'-AAACCCACACATG-3' GRFI site, which are involved in the regulation of expression. OPI1 and SIN3 play the role of repressors for OPI3 expression whereas UME6 is an activator of OPI3 expression. Present with 5890 molecules/cell in log phase SD medium. Belongs to the class VI-like SAM-binding methyltransferase superfamily. PEMT/PEM2 methyltransferase family.