The two major dimethylsulfonium compounds known in higher plants and marine algae are S-methylmethionine (SMM) and 3-dimethylsulfoniopropionate (DMSP). SMM is widely distributed in higher plants and algae, typically at moderate levels; it is not clear whether SMM has any physiological funtion. DMSP occurs only in certain higher plants and marine algae, but can reach very high levels, particularly in conditions of high salinity and low nitrogen. DMSP is a sulfur analog of a betaine and, like betaines, functions as an osmoprotectant. Both SMM and DMSP are known to be precursors of dimethylsulfide (DMS) gas, which is the predominant biological source of atmospheric sulfur. SMM is probably the principal precursor of terrestial DMS emissions, whereas DMSP is the main DMS preursor in the oceans and coastal environments. Both compounds are therefore central to biogenic sulfur emission. Despite the pivotal roles of SMM and DMSP in the natural sulfur cycle, until recently little was known about their biosynthesis except that both are derived from methionine. The overall aims of this research are to elucidate the steps in DMSP biosynthesis in marine algae as well as in higher plants, to understand the enzymes that produce and cleave SMM and how these are kept apart, and to seek the enzymes that convert SMM to DMSP. The results would provide biochemical insight into the origin of the world's most abundant biogenic sources of atmospheric sulfur. This is a collaborative project with Dr. Gage's laboratory at Michigan State University.
