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Dihydropteridine Reductase as an Alternative to Dihydrofolate Reductase for Synthesis of Tetrahydrofolate in Thermus thermophilus

Valérie Wilquet,^1, Mark Van de Casteele,^2, Daniel Gigot,¹ Christianne Legrain,³ and Nicolas Glansdorff^2,3*

Microbiology, Université Libre de Bruxelles,¹ Microbiology, Vrije Universiteit Brussel,² J. M. Wiame Institute for Microbiological Research, B-1070 Brussels, Belgium³

Received 9 June 2003/ Accepted 10 October 2003

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 [DH_Tt] 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). DH_Tt 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.

Present address: Department of Human Genetics, KULeuven/VIB, B-3000 Leuven, Belgium.

Present address: Diabetes Research Center, Vrije Universiteit Brussel, B-1090 Brussels, Belgium.

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