Escherichia colimutants defective in trimethylamineN-oxide reductase

Abstract

Trimethylamine N-oxide (TMAO) is widespread in marine fish and invertebrates [1]. Enteric bacteria reduce TMAO to trimethylamine. Stimulation of anaerobic growth by TMAO has been observed in Escherichia coli [2], Salmonella typhimurium [3] and Proteus sp. strain NTHC153 [4]. According to the chemiosmotic theory, electron flow through the respiratory chain is accompanied by electrogenic extrusion of protons [5]. Proton extrusion and generation of membrane potential coupled to TMAO reduction is observed in anaerobically grown E. coli [6]. Thus, like nitrate reductase and fumarate reductase, TMAO reductase is a terminal electron acceptor for respiratory electron flow. Both TMAO reductase and nitrate reductase accept electrons from NADH, formate or viologen dyes. Defects of TMAO reductase activity in chlD mutants suggest that both TMAO reductase and nitrate reductase use the molybdenum cofactor in E. coli [6,7] and S. typhimurium [3]. However, TMAO reductase activity is found in chlC, chlE and chlG mutants, indicating that the enzyme does not share any polypeptides with nitrate reductase [6,7]. In this study, we isolated and characterized mutants of E. coli defective not in nitrate reductase but in TMAO reductase. We found that the mutation affecting TMAO reductase activity is located in a different gene from chl. 2.1. Bacterial strains and growth conditions