Cell free extracts of Methanobacterium thermoautrophicum were found to catalyze the reduction of CDP and GDP provided that the cells were harvested, disrupted and the extracts assayed under strictly anaerobic conditions. Ribonucleotide reduction is not affected by adenosylcobalamin and cell extracts do not catalyze the tritium exchange reaction between [5-3H2]adenosylcobalamin and water. The reduction of CDP requires ATP and that of GDP requires dTTP as positive allosteric effectors. In contrast, dATP at low concentration stimulates CDP reduction while at higher concentrations it inhibits the reductase reaction.
