Methanobacterium thermoautotrophicum grown on mineral medium contains 120 nmol of Coa-(5-hydroxybenzimidazolyi)cobamides (derivaties of factor HI) per g of dry cell mass as the sole cobamide. The bacterium assimilated several corrinoids and bemzinidazole bases during autotrophic growth. The corrinoids were converted into factor Im; however, after three transfers in 5,6-dimethylbenzimidazole (200 uM)-supplemented mineral medium, derivatives of factor III were completely replaced by derivatives of vitamin B12, which is atypical for methanogens. The total cobamide content of these cells and their growth rate were not affected compared with factor I-containing cells. Therefore, the high cobamide content rather than a particular type of cobamide is required for metabolism of methanogens. Derivatives of factor m are not essential cofactors of cobamide-containing enzymes from methanogenic bacteria, but they are the result of a unique biosynthetic ability of these archaebacteria. The cobamide biosynthesis include unspecific enzymes, which made it possible either to convert non-species-derived comnoids into derivatives of factor m or to synthesize other types of cobamides than factor m. The cobamide biosynthesis is regulated by its end product. In addition, the uptake of extracellular cobamides is controlled, and the assimilated corrinoids regulate cellular cobamide biosynthesis. 124 Bq/nmol) in 0.5 ml of 2 M HCl. The solutions were incubated at 130°C for 2 h. After addition of 1 ml of concentration ammonia, the samples were twice flash evaporated to dryness and finally suspended in 500 ,ul of 80% chloroform-20% methanol. The 14C-labeled bases were then 3076 JOURNAL