Class I ribonucleotide reductaseRibonucleotide reductase (RNR) catalyzes the reductive synthesis of deoxyribonucleotides from their corresponding ribonucleotides. It provides the precursors necessary for DNA synthesis. RNRs are separated into three classes based on their metallocofactor usage. Class I RNRs, found in eukaryotes, bacteria, and many viruses, use a diiron-tyrosyl radical. Class II RNRs, found in bacteria, bacteriophage, algae and archaea, use coenzyme B12 (adenosylcobalamin, AdoCbl). Class III RNRs, found in anaerobic bacteria, bacteriophages, and archaea, use an FeS cluster and S-adenosylmethionine to generate a glycyl radical. Many organisms have more than one class of RNR present in their genomes. All three RNRs have a ten-stranded alpha-beta barrel domain that is structurally similar to the domain of PFL (pyruvate formate lyase). Class I RNR is oxygen-dependent and can be subdivided into classes Ia (eukaryotes, prokaryotes, viruses and phages) and Ib (which is found in prokaryotes only). It is a tetrameric enzyme of two alpha and two beta subunits; this model covers the major part of the alpha or large subunit, called R1 in class Ia and R1E in class Ib.