In Vivo Characterization of Dimethylsulfoniopropionate Lyase in the Fungus Fusarium lateritium.

A fungus, Fusarium lateritium, with dimethylsulfoniopropionate (DMSP) lyase activity was isolated from both seawater and a salt marsh due to its ability to grow on DMSP (with the evolution of dimethyl sulfide) as the sole source of carbon. This is the first reported case of DMSP lyase activity in a fungus. Several other common fungal genera tested did not have DMSP lyase activity. DMSP was taken up more rapidly by F. lateritium than it was utilized, leading to its intracellular accumulation. Inhibitor studies with nystatin and cyanide indicated that DMSP uptake was an energy-dependent process. The lyase was inducible by its substrate, DMSP (K(m), 1.2 mM), and by the substrate analogs choline and glycine betaine. During induction, DMSP lyase activity increased with time and then dropped rapidly. This loss of activity could be prevented by spiking the culture with fresh DMSP or choline. The V(max) for DMSP lyase was 34.7 mU . mg of protein. The inhibitory effects of nystatin, and p-chloromercuriphenylsulfonate on DMSP lyase activity suggested that the enzyme is cytosolic. Because plants like Spartina (a marsh grass) and marine algae contain high concentrations of DMSP, we speculate that DMSP-utilizing fungi may be involved in their decay.

Identification of Pseudomonas aeruginosa glpM, whose gene product is required for efficient alginate biosynthesis from various carbon sources.

In a mucB (algN) genetic background, insertion of an omega element approximately 200 bp downstream of glpD, encoding sn-glycerol-3-phosphate dehydrogenase from Pseudomonas aeruginosa, had an adverse effect on alginate biosynthesis from various carbon sources. The insertion inactivated glpM, a gene encoding a 12,040-M(r) hydrophobic protein containing 109 amino acids. This protein, which was expressed in a T7 RNA polymerase expression system, appears to be a cytoplasmic membrane protein.

Release of dimethylsulfide from dimethylsulfoniopropionate by plant-associated salt marsh fungi.

The range of types of microbes with dimethylsulfoniopropionate (DMSP) lyase capability (enzymatic release of dimethylsulfide [DMS] from DMSP) has recently been expanded from bacteria and eukaryotic algae to include fungi (a species of the genus Fusarium [M. K. Bacic and D. C. Yoch, Appl. Environ. Microbiol. 64:106-111, 1998]). Fungi (especially ascomycetes) are the predominant decomposers of shoots of smooth cordgrass, the principal grass of Atlantic salt marshes of the United States. Since the high rates of release of DMS from smooth cordgrass marshes have a temporal peak that coincides with peak shoot death, we hypothesized that cordgrass fungi were involved in this DMS release. We tested seven species of the known smooth cordgrass ascomycetes and discovered that six of them exhibited DMSP lyase activity. We also tested two species of ascomycetes from other DMSP-containing plants, and both were DMSP lyase competent. For comparison, we tested 11 species of ascomycetes and mitosporic fungi from halophytes that do not contain DMSP; of these 11, only 3 were positive for DMSP lyase. A third group tested, marine oomycotes (four species of the genera Halophytophthora and Pythium, mostly from mangroves), showed no DMSP lyase activity. Two of the strains of fungi found to be positive for DMSP lyase also exhibited uptake of DMS, an apparently rare combination of capabilities. In conclusion, a strong correlation exists between a fungal decomposer's ability to catabolize DMSP via the DMSP lyase pathway and the host plant's production of DMSP as a secondary product.