Escherichia coli contains a specific ribonucleotide reductase for deoxyribonucleotide synthesis and growth under anaerobiosis. The α2β2 enzyme contains an iron-sulfur center on its small β2 subunit that is involved in the one-electron reduction of S-adenosylmethionine and in the generation of a glycyl radical on the large α2 subunit. By a variety of spectroscopic methods (light absorption, resonance Raman, and Mossbauer spectroscopy) and metal and sulfide analysis, it is shown that the metal center is a (2Fe-2S)2+ cluster. Reduction by photoreduced deazaflavin or dithionite converts these centers mostly into (4Fe-4S) clusters, in both the (4Fe-4S)1+ and + states, as is unambiguously demonstrated by Mossbauer spectroscopy at 4.2 and 77 K, in the presence of small (10 mT) or high (5.3 or 7 T) fields. The structure and the function of the iron-sulfur center of the anaerobic ribonucleotide reductase are discussed in relation with other members of a class of enzymes with similar metal centers and functions (reduction of S-adenosylmethionine).