One of the two nonidentical subunits of Escherichia coli ribonucleotide reductase, protein B2, contains in its active form two antiferromagnetically coupled Fe(III) ions and an organic free radical that arises by the one-electron oxidation of tyrosine-122 of the polypeptide chain. Protein B2 lacking the tyrosine radical but with the iron center intact (called protein B2/HU because it is produced by treatment with hydroxyurea) is enzymatically inactive. Previously, it was found that a crude extract from E. coli transforms B2/HU into B2 in the presence of dithiothreitol, Mg2+, and oxygen. On purification of the enzyme system, we now find that radical introduction requires three separate proteins as well as NADPH and FMN. One of the proteins is superoxide dismutase. We hypothesize that the overall reaction involves a reduction of the iron center followed by the oxidation of iron and tyrosine-122. Superoxide dismutase may then be involved in the second step to protect an oxidation-sensitive intermediate. Alternatively, the enzyme might be directly involved in the oxidation step.