FIG 2.

Selected folate cycle enzymes. The folP gene encodes dihydropteroate synthase (DHPS); this protein forms 7,8-dihydropteroate from 6-hydroxymethyl-dihydropterin diphosphate and p-amino benzoate. The folC gene encodes folylpolyglutamate synthase, which adds l-glutamate to dihydropteroate to form 7,8-dihydrofolate (DHF). The folA gene encodes dihydrofolate reductase (DHFR), which catalyzes the NADPH-dependent reduction of DHF to 5,6,7,8-tetrahydrofolate (THF). The glyA gene encodes serine hydroxymethyl transferase (SHMT), which interconverts l-serine and glycine using THF and 5,10-methylene-THF and pyridoxal phosphate as a cofactor. The metF gene encodes methylene-tetrahydrofolate reductase (MTHFR), which reduces 5,10-methylene-THF to 5-methyl-THF using NADPH as a cofactor. The thyA gene encodes thymidylate synthase (TS); this enzyme catalyzes the reductive methylation of 2′-deoxyuridine-5′-monophosphate (dUMP) using 5,10-methylene-THF to produce thymidine-5′-monophosphate (dTMP) and DHF. The metH gene encodes methionine synthase, which forms methionine and THF from 5-methy-THF and homocysteine. The in vivo activities of the colored enzymes were examined by our osmotic stress approach.