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One Pathway Can Incorporate either Adenine or Dimethylbenzimidazole as an -Axial Ligand of B₁₂ Cofactors in Salmonella enterica

Section of Microbiology, University of California, Davis, California 95616,¹ Department of Molecular Biosciences, School of Veterinary Medicine, University of California, Davis, California 95616,² Department of Nutrition, University of California, Davis, California 95616-5270,³ Institute of Organic Chemistry and Centre of Molecular Biosciences, University of Innsbrück, A-6020 Innsbrück, Austria,⁴ Department of Entomology and Cancer Research Center, University of California, Davis, California 95616⁵

Received 26 August 2007/ Accepted 23 October 2007

Corrinoid (vitamin B₁₂-like) cofactors contain various -axial ligands, including 5,6-dimethylbenzimidazole (DMB) or adenine. The bacterium Salmonella enterica produces the corrin ring only under anaerobic conditions, but it can form "complete" corrinoids aerobically by importing an "incomplete" corrinoid, such as cobinamide (Cbi), and adding appropriate - and β-axial ligands. Under aerobic conditions, S. enterica performs the corrinoid-dependent degradation of ethanolamine if given vitamin B₁₂, but it can make B₁₂ from exogenous Cbi only if DMB is also provided. Mutants isolated for their ability to degrade ethanolamine without added DMB converted Cbi to pseudo-B₁₂ cofactors (having adenine as an -axial ligand). The mutations cause an increase in the level of free adenine and install adenine (instead of DMB) as an -ligand. When DMB is provided to these mutants, synthesis of pseudo-B₁₂ cofactors ceases and B₁₂ cofactors are produced, suggesting that DMB regulates production or incorporation of free adenine as an -ligand. Wild-type cells make pseudo-B₁₂ cofactors during aerobic growth on propanediol plus Cbi and can use pseudo-vitamin B₁₂ for all of their corrinoid-dependent enzymes. Synthesis of coenzyme pseudo-B₁₂ cofactors requires the same enzymes (CobT, CobU, CobS, and CobC) that install DMB in the formation of coenzyme B₁₂. Models are described for the mechanism and control of -axial ligand installation.

Published ahead of print on 2 November 2007.