A gene, 'cobA' + 'hemD', from 'Selenomonas ruminantium' encodes a bifunctional enzyme involved in the synthesis of vitamin B₁₂

Coenzymes derived from vitamin B₁₂ (cyanocobalamin) are particularly important for core metabolism in ruminant animals. 'Selenomonas ruminantium', a Gram-positive obligate anaerobe isolated from cattle, is the main contributor of vitamin B₁₂ to such ruminant animals. In nature, there are both aerobic and anaerobic pathways for B₁₂ synthesis - the latter is only partly elucidated. Until now, there has been no investigation of B₁₂ synthesis in 'S. ruminantium', which must use an anaerobic pathway. This paper reports the cloning of the chromosomal operon from 'S. ruminantium' which is responsible for the first committed steps in corrinoid synthesis. Five open reading frames were found in the cloned fragment. All deduced amino acid sequences had similarity to defined proteins in the databases that are involved in porphyrin and corrin synthesis. Of particular interest is the gene designated 'cobA' + 'hemD', which encodes a single polypeptide possessing two catalytic functions - uroporphyrinogen III synthase and uroporphyrinogen III 2,7-methyltransferase. This enzyme converts hydroxymethylbilane to precorrin-2. The functions of the protein coded by 'cobA' + 'hemD' were established by heterologous expression in 'Escherichia coli'. The CobA activity has been demonstrated for three distinct types of proteins - monofunctional, bifunctional with siroheme formation and, this report, bifunctional with uroporphyrinogen III synthesis. The type found in 'S. ruminantium' ('cobA' + 'hemD') is probably restricted to obligately anaerobic fermentative bacteria.