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J. Biol. Chem., Vol. 276, Issue 13, 9587-9589, March 30, 2001

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ACCELERATED PUBLICATION
Activation of Class III Ribonucleotide Reductase by Thioredoxin^*

Dominique Padovani, Etienne Mulliez, and Marc Fontecave^§

From the Laboratoire de Chimie et Biochimie des Centres Rédox Biologiques, Département de Biologie Moléculaire et Structurale-Chimie Biologie Commissariat à l'Energie Atomique/CNRS/Université Joseph Fourier, 17 avenue des Martyrs, 38054 Grenoble, Cedex 09, France

Anaerobic ribonucleotide reductase provides facultative and obligate anaerobic microorganisms with the deoxyribonucleoside triphosphates used for DNA chain elongation and repair. In Escherichia coli, the dimeric 2 enzyme contains, in its active form, a glycyl radical essential for the reduction of the substrate. The introduction of the glycyl radical results from the reductive cleavage of S-adenosylmethionine catalyzed by the reduced (4Fe-4S) center of a small activating protein called . This activation reaction has long been known to have an absolute requirement for dithiothreitol. Here, we report that thioredoxin, along with NADPH and NADPH:thioredoxin oxidoreductase, efficiently replaces dithiothreitol and reduces an unsuspected critical disulfide bond probably located on the C terminus of the  protein. Activation of reduced  protein does not require dithiothreitol or thioredoxin anymore, and activation rates are much faster than previously reported. Thus, in E. coli, thioredoxin has very different roles for class I ribonucleotide reductase where it is required for the substrate turnover and class III ribonucleotide reductase where it acts only for the activation of the enzyme.

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