RNRdb, a curated database of the universal enzyme family ribonucleotide reductase, reveals a high level of misannotation in sequences deposited to Genbank

Table 1 General characteristics of RNR classes

	Class I	Class II	Class III
Operation	Aerobic	Oxygen independent	Anaerobic
Structure	α₂ β₂	α or α₂	α₂
Subunit names	Ia: NrdA, NrdB	NrdJ	NrdD
	Ib: NrdE, NrdF		specific activase: NrdG
Radical/cofactor	Tyr122 (in β)	AdoCbl	Gly580 (in α)
Reductant	Ia: Thioredoxin/Glutaredoxin	Thioredoxin	Formate
	Ib: NrdH-redoxin
In archaea	Limited distribution	Yes	Yes
In bacteria	Yes	Yes	Yes
In eukaryotes	Yes	Limited distribution	Limited distribution
In viruses^a	Yes	Bacteriophage; one eukaryotic virus	Bacteriophage

The catalytic subunits, NrdA, NrdE, NrdJ and NrdD are homologous, as are the two different class I radical generating subunits, NrdB and NrdF. The class III activase, NrdG, is not homologous to any other RNR subunit. Residue numbering refers to the Escherichia coli K12 class I radical generating subunit and the Bacteriophage T4 class III enzyme sequences respectively. Adapted from [2].
^aRNR genes are only found in some dsDNA viruses.

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