On the biosynthesis of 5-methoxybenzimidazole. Precursor-function of 5-hydroxybenzimidazole, benzimidazole and riboflavin.

1. In Clostridium thermoaceticum 5-hydroxybenzimidazole is methylated to 5-methoxybenz-imidaxole and transformed to 5-methoxybenzimidazolylcobamide. 5-Hydroxybenzimidazolycobamide is also methylated to 5-methoxybenzimidazolylcobamide. These results indicate a possible precursor function of 5-hydroxybenzimidazole in the biosynthesis of 5-methoxybenzimidazole. 2. The same microorganism uses benzimidazole to form benzimidazolylcobamide. This or externally added benzimidazolylcobamide, although taken up by the cells, is not further transformed (i.e. hydroxylated and methylated to 5-methoxybenzimdazolylcobamidel). This excludes a precursor function of benzimidazole in the biosynthesis of 5-methoxybenzimidazole. 3. Contrary to the biosynthesis of 5,6-dimethylbenzimidazole, 5-methoxybenzimidazole is not formed from riboflavin, but riboflavin inhibits the growth and the production of 5-methoxybenzimidazolylcobamide in Clostridium thermoaceticum. A tentative scheme for the biosynthesis of 5-methoxybenzimidazole via a riboflavin analog is discussed.