A possible new class of ribonucleotide reductase from Methanobacterium thermoautotrophicum

The ribonucleotide reductase from the strictly anaerobic methanogen Methanobacterium thermoautotrophicum has been partially purified by ion-exchange and gel-filtration chromatography. Its molecular weight is estimated to be 100,000 by the latter step. Unlike all previously studied ribonucleotide reductases, the enzyme does not employ dithiol compounds such as dithiothreitol as artificial electron donors in in vitro assays. Inhibition of the enzyme by S-adenosylmethionine, oxygen, and azide further distinguishes it from the Escherichia coli anaerobic enzyme, the iron- and manganese-containing, and the adenosylcobalamin-dependent enzymes. Our preliminary results suggest that this enzyme has an activation mechanism different from the known classes of ribonucleotide reductases.