Methyltransferase that mediates the transfer of a N5-methyl group of (6S)-methyltetrahydrofolate to the 5-methoxybenzimidazolylcobamide cofactor of a corrinoid/Fe-S protein (AcsC/AcsD) in the anaerobic acetyl-CoA pathway (Wood-Ljungdahl pathway) of carbon monoxide and carbon dioxide fixation. (6S)-5,6,7,8-tetrahydrofolate + methyl-Co(III)-[corrinoid Fe-S protein] = (6S)-5-methyl-5,6,7,8-tetrahydrofolate + Co(I)-[corrinoid Fe-S protein] + H(+) Heterohexamer composed of 2 subunits of AcsC, 2 subunits of AcsD and 2 subunits of AcsE. Belongs to the vitamin-B12 dependent methionine synthase family.