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Mycobacterium tuberculosis is included among a select group of bacteria possessing the capacity for de novo biosynthesis of vitamin B₁₂, the largest and most complex natural organometallic cofactor. The bacillus is also able to scavenge B₁₂ and related corrinoids utilizing an ATP-binding cassette-type protein that is distinct from the only known bacterial B₁₂-specific transporter, BtuFCD. Consistent with the inferred requirement for vitamin B₁₂ for metabolic function, the M. tuberculosis genome encodes two B₁₂ riboswitches and three B₁₂-dependent enzymes. Two of these enzymes have been shown to operate in methionine biosynthesis (MetH) and propionate utilization (MutAB), while the function of the putative nrdZ-encoded ribonucleotide reductase remains unknown. Taken together, these observations suggest that M. tuberculosis has the capacity to regulate core metabolic functions according to B₁₂ …