Trimethylammonia-lyases of Shewanella oneidensis and their role in anaerobic respiration.

The facultatively anaerobic bacterium Shewanella oneidensis MR-1 contains two operons in its genome, so_3056-3058 and so_3299-3301, each comprising genes encoding a putative periplasmic flavocytochrome c and a putative periplasmic aromatic amino acid ammonia-lyase. 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 the trimethylammonium group from ergothioneine to yield thiourocanic acid, whereas SO_3299 catalyzed a similar conversion of N(π)-methylhistidine betaine to yield N(π)-methylurocanate. The catalytic efficiencies (kcat/Km values) were (3-4) × 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 stimulated cell growth, while thiourocanate supported respiration by serving as an alternative terminal electron acceptor. The predicted 3D structures of the genetically coupled flavocytochromes c (SO_3056/3058 and SO_3300/3301) are consistent with their use of thiourocanate and N(π)-methylurocanate, 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.