TABLE 2.

S. aureus thioredoxin TrxA and NrdH function with the thioredoxin reductase TrxB as hydrogen donors for the class Ib ribonucleotide reductase NrdEF^a

Hydrogen donor system	dCDP formed (mU)	Sp act (U/μg protein)
None	9	0.4
TrxB	39	1.8
TrxA + TrxB	213	9.7
NrdH + TrxB	168	7.6
NrdH + GSH + GR	ND	ND

^aRNR assays were performed with [³H]CDP as a substrate and 15 μg NrdE, 7 μg NrdF, and 0.4 mM NADPH as the source of reducing power. The reaction mixtures contained 5 μg thioredoxin (TrxA) plus 1 μg thioredoxin reductase (TrxB), 4.5 μg of the NrdH protein plus 1 μg thioredoxin reductase, or 1 mM glutathione (GSH) plus 6 μg glutathione reductase (GR) as hydrogen donors. mU, milliunits; ND, not detectable.