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Abstract
Dimethylsulfoniopropionate (DMSP) is an abundant organosulfur compound in marine surface water that is processed by marine bacteria by cleavage and demethylation pathways. The demethylation pathway releases methanethiol, while the cleavage pathway produces acrylate and the climatically-active gas dimethylsulfide (DMS). When methanethiol is oxidized by the oxidase MtoX, it produces hydrogen peroxide (H2O2), leading to oxidative stress. Previous studies reported that DMSP and its cleavage products formed an antioxidant system in algae, corals, and some higher plants, all of which process DMSP by the cleavage pathway. To study how oxidative stress affects DMSP metabolism in bacteria, Ruegeria pomeroyi and its catalase deletion mutant were treated with H2O2 in a chemostat growing on glucose plus or minus DMSP. The presence of DMSP protected the mutant against H2O2. RNA-seq analysis indicated that under oxidative stress, the demethylation pathway was downregulated, but the cleavage pathway was upregulated. These results confirmed the antioxidant role of DMSP in bacteria and revealed oxidative stress is a factor regulating DMSP metabolism in R. pomeroyi.DmdC is the 3-methylmercaptopropionyl-CoA (MMPA-CoA) dehydrogenase catalyzing the third reaction of the demethylation pathway. DmdC1 in R. pomeroyi was recombinantly expressed and characterized for kinetic properties. Among the short chain acyl-CoAs tested, MMPA-CoA was the best substrate. DmdC1 was not affected by potential effectors at physiological concentrations, including DMSP, MMPA, ATP and ADP. These findings suggest that DmdC1 has only minimal adaptations for DMSP metabolism, supporting the hypothesis that it evolved relatively recently from a short chain acyl-CoA dehydrogenase involved in fatty acid oxidation.
The roseobacter group is a paraphyletic group within the family Rhodobacteraceae in the Alphaproteobacteria. Members in this group are abundant in marine environments and highly diverse physiologically and genetically. Recent research indicated that 16S rRNA gene sequence similarity is a poor marker for the phylogeny within this group. By whole-genome sequence based analysis, most species in the genus Loktanella were moved into several novel genera. After reanalyzing the phylogeny, Loktanella ponticola, whose genome became available recently, and a newly isolated species Lotanella acticola were moved into the genus Yoonia.