Key enzyme in folate metabolism. Catalyzes an essential reaction for de novo glycine and purine synthesis, and for DNA precursor synthesis. (6S)-5,6,7,8-tetrahydrofolate + NADP(+) = 7,8-dihydrofolate + H(+) + NADPH Cofactor biosynthesis; tetrahydrofolate biosynthesis; 5,6,7,8-tetrahydrofolate from 7,8-dihydrofolate: step 1/1. The strain K12 sequence is shown. Strain B [RT500] is resistant to 500 micrograms per milliliter of trimethoprim. Strain B [MB1428] is methotrexate-resistant. Belongs to the dihydrofolate reductase family.