Microbial dimethylsulfoxide and trimethylamine-N-oxide respiration.

Over the last two decades, the biochemistry and genetics of dimethylsulfoxide (DMSO) and trimethylamine-N-oxide (TMAO) respiration has been characterised, particularly in Escherichia coli marine bacteria of the genus Shewanella and the purple phototrophic bacteria, Rhodobacter sphaeroides and R. capsulatus. All of the enzymes (or catalytic subunits) involved the final step in DMSO and TMAO respiration contain a pterin molybdenum cofactor and are members of the DMSO reductase family of molybdoenzymes. In E. coli, the dimethylsulfoxide reductase (DmsABC) can be purified from membranes as a complex, which exhibits quinol-DMSO oxidoreductase activity. The enzyme is anchored to the membrane via the DmsC subunit and its catalytic subunit DmsA is now considered to face the periplasm. Electron transfer to DmsA involves the DmsB subunit, which is a polyferredoxin related to subunits found in other molybdoenzymes such as nitrate reductase and formate dehydrogenase. A characteristic of the DmsAB-type DMSO reductase is its ability to reduce a variety of S- and N-oxides. E. coli contains a trimethylamine-N-oxide reductase (TorA) that is highly specific for N-oxides. This enzyme is located in the periplasm and is connected to the quinone pool via a membrane-bound penta-haem cytochrome (TorC). DorCA in purple phototrophic bacteria of the genus Rhodobacter is very similar to TorCA with the critical difference that DorA catalyses reduction of both DMSO and TMAO. It is known as a DMSO reductase because the S-oxide is the best substrate. Crystal structures of DorA and TorA have revealed critical differences at the Mo active site that may explain the differences between substrate specificity between the two enzymes. DmsA, TorA and DorA possess a "twin arginine" N-terminal signal sequence consistent with their secretion via the TAT secretory system and not the Sec system. The enzymes are secreted with their bound prosthetic groups: this take place in the cytoplasm and the biogenesis involves a chaperone protein, which is cognate for each enzyme. Expression of the DMSO and TMAO respiratory operons is induced in response to a fall in oxygen tension. dmsABC expression is positively controlled by the oxygen-responsive transcription factor, Fnr and ModE, a transcription factor that binds molybdate. In contrast, torCAD expression is not under Fnr- or ModE-control but is dependent upon a sensor histidine kinase-response regulator pair, TorSR, which activate gene expression under conditions of low oxygen tension in the presence of N- or S-oxide. Regulation of dorCDA expression is similar to that seen for torCAD but it appears that the expression of the sensor histidine kinase-response regulator pair, DorSR is regulated by Fnr and there is an additional tier of regulation involving the ModE-homologue MopB, molybdate and the transcription factor DorX. Analysis of microbial genomes has revealed the presence of dms and tor operons in a wide variety of bacteria and in some archaea and duplicate dms and tor operons have been identified in E. coli. Challenges ahead will include the determination of the significance of the presence of the dms operon in bacterial pathogens and the determination of the significance of DMSO respiration in the global turnover of marine organo-sulfur compounds.