One pathway can incorporate either adenine or dimethylbenzimidazole as -axial ligand of B₁₂ cofactors in Salmonella enterica

Section of Microbiology, University of California, Davis, California, 95616, USA; Department of Molecular Biosciences, School of Veterinary Medicine, University of California, Davis, California, 95616, USA; Department of Nutrition, University of California, Davis, California, 95616-5270, USA; Institute of Organic Chemistry & Centre of Molecular Biosciences, University of Innsbrück, A-6020 Innsbrück, Austria; Department of Entomology & Cancer Research Center, University of California, Davis, California, 95616, USA

^* To whom correspondence should be addressed. Email: Peter.Anderson{at}med.monash.edu.au.

	Abstract

Corrinoid (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 can form "complete" corrinoids aerobically by importing an "incomplete" corrinoid such as cobinamide (Cbi) and adding appropriate - and -axial ligands. Under aerobic conditions, S. enterica performs the corrinoid-dependent degradation of ethanolamine if given B₁₂, but 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 -axial ligand). The mutations cause an increase in the level of free adenine and install adenine (instead of DMB) as -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 -ligand. Wild type cells make pseudo-B₁₂ cofactors during aerobic growth on propanediol plus Cbi and can use pseudovitamin-B₁₂ for all of their corrinoid-dependent enzymes. Synthesis of coenzyme pseudo-B₁₂ cofactors requires the same enzymes (CobT, U, S, C) that install DMB in formation of coenzyme B₁₂. Models are described for the mechanism and control of -axial ligand installation.