This enzyme, characterized from the anaerobic bacterium Desulfitobacterium hafniense Y51, catalyses a similar reaction to that of EC 2.1.1.258, 5-methyltetrahydrofolate---corrinoid/iron-sulfur protein Co-methyltransferase, but in the opposite direction, transferring a methyl group from a methylated corrinoid protein to tetrahydrofolate. The corrinoid protein is specifically methylated by EC 2.1.1.376, glycine betaine---corrinoid protein Co-methyltransferase.