Catalyzes oxidation of cytoplasmic one-carbon units for purine biosynthesis. (6R)-5,10-methylene-5,6,7,8-tetrahydrofolate + NAD(+) = 5,10-methenyltetrahydrofolate + NADH Homodimer. The N-terminus is blocked. Present with 16200 molecules/cell in log phase SD medium. Belongs to the tetrahydrofolate dehydrogenase/cyclohydrolase family.