Anaerobic growth of Paracoccus denitrificans requires cobalamin: Characterization of cobK and cobJ genes

A pleiotropic mutant of Paracoccus denitrificans, which has a severe defect that affects its anaerobic growth when either nitrate, nitrite, or nitrous oxide is used as the terminal electron acceptor and which is also unable t o use ethanolamine as a carbon and energy source for aerobic growth, was is olated. This phenotype of the mutant is expressed only during growth on min imal media and can be reversed by addition of cobalamin (vitamin B-12) or c obinamide to the media or by growth on rich media. Sequence analysis reveal ed the mutation causing this phenotype to be in a gene homologous to cobK o f Pseudomonas denitrificans, which encodes precorrin-6x reductase of the co balamin biosynthesis pathway. Convergently transcribed with cobK is a gene homologous to cobJ of Pseudomonas denitrificans, which encodes precorrin-3b methyltransferase, The inability of the cobalamin auxotroph to grow aerobi cally on ethanolamine implies that wild-type P. denitrificans (which can gr ow on ethanolamine) expresses a cobalamin-dependent ethanolamine ammonia ly ase and that this organism synthesizes cobalamin under both aerobic and ana erobic growth conditions. Comparison of the cobK and cobJ genes with their orthologues suggests that P. denitrificans uses the aerobic pathway for cob alamin synthesis. It is paradoxical that under anaerobic growth conditions, P, denitrificans appears to use the aerobic (oxygen-requiring) pathway for cobalamin synthesis. Anaerobic growth of the cobalamin auxotroph could be restored by the addition of deoxyribonucleosides to minimal media. These ob servations provide evidence that P, denitrificans expresses a cobalamin-dep endent ribonucleotide reductase, which is essential for growth only under a naerobic conditions.