Cytoplasmic isozyme of C-1-tetrahydrofolate synthase. The trifunctional enzyme catalyzes the interconversion of the one-carbon derivatives of tetrahydrofolate (THF) between different oxidation states by the enzymatic activities 10-formyltetrahydrofolate synthetase, 5,lO-methenyltetrahydrofolate cyclohydrolase, and 5,lO-methylenetetrahydrofolate dehydrogenase. Involved in the generation of one-carbon intermediates in the biosynthesis of the purine bases. (6R)-5,10-methylene-5,6,7,8-tetrahydrofolate + NADP(+) = 5,10-methenyltetrahydrofolate + NADPH 5,10-methenyltetrahydrofolate + H2O = (6S)-10-formyltetrahydrofolate + H(+) (6S)-5,6,7,8-tetrahydrofolate + ATP + formate = (6S)-10-formyltetrahydrofolate + ADP + phosphate One-carbon metabolism; tetrahydrofolate interconversion. Homodimer. This trifunctional enzyme consists of two major domains: an N-terminal part containing the methylene-THF dehydrogenase and cyclohydrolase activities and a larger C-terminal part containing formyl-THF synthetase activity. Present with 35600 molecules/cell in log phase SD medium. In the N-terminal section; belongs to the tetrahydrofolate dehydrogenase/cyclohydrolase family. In the C-terminal section; belongs to the formate--tetrahydrofolate ligase family.