A vitamin B-12 transporter in Mycobacterium tuberculosis

Vitamin B-12-depende enzymes function in core biochemical pathways in Mycobacterium tuberculosis, an obligate pathogen whose metabolism in vivo is poorly understood. Although M. tuberculosis can access vitamin B-12 in vitro, it is uncertain whether the organism is able to scavenge B-12 during host infection. This question is crucial to predictions of metabolic function, but its resolution is complicated by the absence in the M. tuberculosis genome of a direct homologue of BtuFCD, the only bacterial B-12 transport system described to date. We applied genome-wide transposon mutagenesis to identify M. tuberculosis mutants defective in their ability to use exogenous B-12. A small proportion of these mapped to Rv1314c, identifying the putative PduO-type ATP : co(I) rrinoid adenosyltransferase as essential for B-12 assimilation. Most notably, however, insertions in Rv1819c dominated the mutant pool, revealing an unexpected function in B-12 acquisition for an ATP-binding cassette (ABC)-type protein previously investigated as the mycobacterial BacA homologue. Moreover, targeted deletion of Rv1819c eliminated the ability of M. tuberculosis to transport B-12 and related corrinoids in vitro. Our results establish an alternative to the canonical BtuCD-type system for B-12 uptake in M. tuberculosis, and elucidate a role in B-12 metabolism for an ABC protein implicated in chronic mycobacterial infection.