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Oxidation of methanethiol (MT) is a significant step in the sulfur cycle. MT is an intermediate of metabolism of globally significant organosulfur compounds including dimethylsulfoniopropionate (DMSP) and dimethylsulfide (DMS), which have key roles in marine carbon and sulfur cycling. In aerobic bacteria, MT is degraded by a MT oxidase (MTO). The enzymatic and genetic basis of MT oxidation have remained poorly characterized. Here, we identify for the first time the MTO enzyme and its encoding gene (mtoX) in the DMS-degrading bacterium Hyphomicrobium sp. VS. We show that MTO is a homotetrameric metalloenzyme that requires Cu for enzyme activity. MTO is predicted to be a soluble periplasmic enzyme and a member of a distinct clade of the Selenium-binding protein (SBP56) family for which no function has been reported. Genes orthologous to mtoX exist in many bacteria able to degrade DMS, other one-carbon compounds or DMSP, notably in the marine model organism Ruegeria pomeroyi DSS-3, a member of the Rhodobacteraceae family that is abundant in marine environments. Marker exchange mutagenesis of mtoX disrupted the ability of R. pomeroyi to metabolize MT confirming its function in this DMSP-degrading bacterium. In R. pomeroyi, transcription of mtoX was enhanced by DMSP, methylmercaptopropionate and MT. Rates of MT degradation increased after pre-incubation of the wild-type strain with MT. The detection of mtoX orthologs in diverse bacteria, environmental samples and its abundance in a range of metagenomic data sets point to this enzyme being widely distributed in the environment and having a key role in global sulfur cycling.

Dimethylsulphide (DMS) plays a major role in the global sulphur cycle. It has important implications for atmospheric chemistry, climate regulation, and sulphur transport from the marine to the atmospheric and terrestrial environments. In addition, DMS acts as an info-chemical for a wide range of organisms ranging from micro-organisms to mammals. Micro-organisms that cycle DMS are widely distributed in a range of environments, for instance, oxic and anoxic marine, freshwater and terrestrial habitats. Despite the importance of DMS that has been unearthed by many studies since the early 1970s, the understanding of the biochemistry, genetics, and ecology of DMS-degrading micro-organisms is still limited. This review examines current knowledge on the microbial cycling of DMS and points out areas for future research that should shed more light on the role of organisms degrading DMS and related compounds in the biosphere.

The oxidation of methane to methanol in methanotrophs is catalyzed by the enzyme methane monooxygenase (MMO). Two distinct forms of this enzyme exist, a soluble cytoplasmic MMO (sMMO) and a membrane-bound particulate form (pMMO). The active protein complex termed pMMO-C was purified recently from Methylococcus capsulatus (Bath). The complex consists of pMMO hydroxylase and an additional component pMMO-R, which was proposed to be the reductase for the pMMO complex. Further study of this complex has led here to the proposal that the pMMO-R is in fact methanol dehydrogenase, the subsequent enzyme in the methane oxidation pathway by methanotrophs. We describe here the biochemical and biophysical characterization of a stable purified complex of pMMO hydroxylase (pMMO-H) with methanol dehydrogenase (MDH) and report the first three-dimensional (3D) structure, determined by cryoelectron microscopy and single particle analysis to approximately 16 A resolution. The 3D structure reported here provides the first insights into the supramolecular organization of pMMO with MDH. These studies of pMMO-MDH complexes have provided further understanding of the structural basis for the particular functions of the enzymes in this system which might also be of relevance to the complete process of methane oxidation by methanotrophs under high copper concentration in the environment.

The oxidation of methane to methanol in methanotrophs is catalyzed by the enzyme methane monooxygenase (MMO). Two distinct forms of this enzyme exist, a soluble cytoplasmic MMO (sMMO) and a membrane-bound particulate form (pMMO). We describe here the biochemical characterization of a stable and active purified pMMO hydroxylase (pMMO-H) and report a three-dimensional (3D) structure, determined by electron microscopy and single-particle analysis at 23 A resolution. Both biochemical and structural data indicate that pMMO hydroxylase is trimeric, with each monomer unit comprised of three polypeptides of 47, 26, and 23 kDa. Comparison of the recent crystal structure [Lieberman, R. L., and Rosenzweig, A. C. (2005) Nature 434, 177] of an uncharacterized pMMO-H complex with the three-dimensional (3D) structure determined here yielded a good match between the principal features and the organization of the enzyme monomers into trimers. The data presented here advance our current understanding of particulate methane monooxygenase function by the characterization of an active form of the enzyme and the corresponding 3D structure.