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. There are two DhfR isozymes in S.aureus, this one is chromosomal and is sensitive to trimethoprim. Belongs to the dihydrofolate reductase family.