Trimethylammonia-lyases of Shewanella oneidensis and Their Role in Anaerobic Respiration

The facultatively anaerobic bacterium Shewanella oneidensis MR-1 contains in its genome two operons, so_3056-3058 and so_3299-3301, each including genes for putative periplasmic flavocytochrome c and ammonia-lyase of aromatic amino acids. To determine their role in anaerobic respiration, we produced the encoded ammonia-lyases SO_3057 and SO_3299 in Escherichia coli and determined their substrate specificities. SO_3057 was found to cleave trimethylammonium group from ergothioneine to yield thiourocanic acid, whereas SO_3299 catalyzed a similar conversion of N({pi})-methyl histidine betaine to yield N({pi})-methyl urocanate. The catalytic efficiencies (kcat/Km values) were (3-4) x 106 M-1 s-1, and the pH optima of activity were between 8 and 9. Ergothioneine induced SO_3057 synthesis in anaerobic S. oneidensis cells and their growth, and thiourocanate stimulated respiration as an alternative terminal electron acceptor. The predicted 3D structures of the genetically coupled flavocytochromes c (SO_3056/58 and SO_3300/3301) are consistent with their use of thiourocanate and N({pi})-methyl urocanate, respectively, as electron acceptors. We therefore conclude that the periplasmic lyases encoded by the so_3057 and so_3299 genes contribute to anaerobic respiration in S. oneidensis by producing terminal electron acceptors for the genetically coupled flavocytochromes c.