Escherichia coli contains a specific ribonucleotide reductase for deoxyribonucleotide synthesis and growth under anaerobiosis. The alpha(2)beta(2) enzyme contains an iron-sulfur center on its small beta(2) subunit that is involved in the one-electron reduction of S-adenosylmethionine and in the generation of a glycyl radical on the large alpha(2) 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 (4Fe-4S)(2+) 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 Ether members of a class of enzymes with similar metal centers and functions (reduction of S-adenosylmethionine).