Vitamin B12 Biosynthesis and Anaerobic Use of Ethanolamine

The ability to synthesize vitamin B12 is unevenly distributed in living organisms, and vitamin B12 biosynthesis genes are found in about one-third of the bacteria sequenced. Vitamin B12 can be synthesized in oxygen-independent and oxygen-dependent pathways [100]. A gene cluster (cbi and cob genes) identified in the genome sequences of L. monocytogenes and L. innocua [2, 64] shares high homology with vitamin B12 biosynthesis genes from Salmonella enterica serovar Typhimur-ium, suggesting that both Listeria species use the oxygen-independent pathway like S. enterica. Furthermore, close to the cobalamin biosynthesis genes both Listeria species contain orthologues of genes necessary in S. enterica for the coenzyme B12-dependent degradation of ethanolamine and propanediol (eut and pdu genes, respectively). All three gene clusters [from cbiP (lmo1208) to pduS (lmo1142)] may have been acquired by Listeria en bloc by horizontal gene transfer in an ancient event [64]. L. monocytogenes is an aerobically growing microaerophilic that thrives best at reduced oxygen tension and is able to colonize and survive in the mammalian gut where it encounters anaerobic conditions [4]. Vitamin B12- dependent anaerobic degradation of ethanolamine and propanediol could enable

L. monocytogenes to use ethanolamine and 1,2-propanediol as carbon and energy sources for growth under the anaerobic conditions encountered in the mammalian gut, where both substances are believed to be abundant. Taken together, it is tempting to speculate that the vitamin B12 synthesis genes together with the pdu and eut operons play a role during a listerial infection and hence may represent a novel type of virulence determinants of L. monocytogenes.

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