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Dihydropteridine Reductase as an Alternative to Dihydrofolate Reductase for Synthesis of Tetrahydrofolate in Thermus thermophilus. Valérie Wilquet, 2004.A strategy devised to isolate a gene coding for a dihydrofolate reductase from Thermus thermophilus DNA delivered only clones harboring instead a gene (the T . thermophilus dehydrogenase [DHTt] gene) coding for a dihydropteridine reductase which displays considerable dihydrofolate reductase activity (about 20% of the activity detected with 6,7-dimethyl-7,8-dihydropterine in the quinonoid form as a substrate) . DHTt appears to account for the synthesis of tetrahydrofolate in this bacterium, since a classical dihydrofolate reductase gene could not be found in the recently determined genome nucleotide sequence (A . Henne, personal communication) . The derived amino acid sequence displays most of the highly conserved cofactor and active-site residues present in enzymes of the short-chain dehydrogenase/reductase family . The enzyme has no pteridine-independent oxidoreductase activity, in contrast to Escherichia coli dihydropteridine reductase, and thus appears more similar to mammalian dihydropteridine reductases, which do not contain a flavin prosthetic group . We suggest that bifunctional dihydropteridine reductases may be responsible for the synthesis of tetrahydrofolate in other bacteria, as well as archaea, that have been reported to lack a classical dihydrofolate reductase but for which possible substitutes have not yet been identified .
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