Class I ribonucleotide reductase (RNR) consists of a catalytic subunit (NrdA) and a radical-generating subunit (NrdB) that together catalyze reduction of ribonucleotides to their corresponding deoxyribonucleotides. NrdB from the firmicute is a unique fusion protein with N-terminal add-ons of a glutaredoxin (Grx) domain followed by an ATP-binding domain， the ATP-cone. Grx， usually encoded separately from the RNR operon， is a known RNR reductant. We show that the fused Grx domain functions as an efficient reductant of the class I RNR via the common dithiol mechanism and， interestingly， also via a monothiol mechanism， although less efficiently. To our knowledge， a Grx that uses both of these two re... More
Class I ribonucleotide reductase (RNR) consists of a catalytic subunit (NrdA) and a radical-generating subunit (NrdB) that together catalyze reduction of ribonucleotides to their corresponding deoxyribonucleotides. NrdB from the firmicute is a unique fusion protein with N-terminal add-ons of a glutaredoxin (Grx) domain followed by an ATP-binding domain， the ATP-cone. Grx， usually encoded separately from the RNR operon， is a known RNR reductant. We show that the fused Grx domain functions as an efficient reductant of the class I RNR via the common dithiol mechanism and， interestingly， also via a monothiol mechanism， although less efficiently. To our knowledge， a Grx that uses both of these two reaction mechanisms has not previously been observed with a native substrate. The ATP-cone is in most RNRs an N-terminal domain of the catalytic subunit. It is an allosteric on/off switch promoting ribonucleotide reduction in the presence of ATP and inhibiting RNR activity in the presence of dATP. We found that dATP bound to the ATP-cone of NrdB promotes formation of tetramers that cannot form active complexes with NrdA. The ATP-cone bound two dATP molecules， but only one ATP molecule. NrdB contains the recently identified radical-generating cofactor Mn/Mn We show that NrdA from can form a catalytically competent RNR with the Mn/Mn-containing NrdB from the flavobacterium In conclusion， NrdB is fused with a Grx functioning as an RNR reductant and an ATP-cone serving as an on/off switch.
