Folate Coenzymes in Amethopterin-sensitive and -resistant Strains of Streptococcus faecalis

Formyltetrahydrofolate synthetase was partially purified from extracts of two amethopterin-resistant mutant strains of Streptococcus faecalis, SF/A and SF/Ak. The concentrations of formate, tetrahydrofolate, adenosine triphosphate, and MgCl2 giving half-maximal reaction velocity were determined. The purified enzyme requires Mg++ for maximal activity and is specific for formate. The product of the enzymatic reaction is 10-formyltetrahydrofolate. Tetrahydropteroyltriglutamate can replace tetrahydrofolate; its Km value is 10 times larger than that of tetrahydrofolate. The pH optimum is 6.4 and is ascorbate dependent; the temperature coefficient (35–45°) is 1.9. Inorganic phosphate and ADP inhibit the reaction. Purines, which repress synthesis of the enzyme, are not strong inhibitors of formate activation. Formyltetrahydrofolate synthetase was not found in the amethopterin-sensitive strain, SF/O. The presence of serine hydroxymethyltransferase, 5,10-methylenetetrahydrofolate dehydrogenase, and 5,10-methenyltetrahydrofolate cyclohydrolase suggest the preferential utilization of exogenous serine as source of active 1 carbon moieties. Why serine synthesis de novo is independent of a formate-activating enzyme in SF/O is not yet clear.