B14C-06
Metabolic Interfaces of Mercury Methylation Proteins in Desulfovibrio sp. ND132
Monday, 14 December 2015: 17:15
2010 (Moscone West)
Judy D. Wall1, Romain Bridou1, Steven D Smith1, Kenny Mok2, Florian Widner2, Alexander Johs3, Jerry Parks4, Eric M Pierce3, Dwayne A Elias5, Cynthia C Gilmour6 and Michi Taga2, (1)University of Missouri Columbia, Columbia, MO, United States, (2)University of California Berkeley, Department of Plant & Microbial Biology, Berkeley, CA, United States, (3)Oak Ridge National Laboratory, Environmental Sciences Division, Oak Ridge, TN, United States, (4)Oak Ridge National Lab, Center for Molecular Biophysics, Oak Ridge, TN, United States, (5)Oak Ridge National Laboratory, Biosciences Division, Oak Ridge, TN, United States, (6)Smithsonian Institution, Smithsonian Environmental Research Center, Edgewater, MD, United States
Abstract:
Two genes necessary for microbial production of the neurotoxin methylmercury have been identified; hgcA encoding a corrinoid methyltransferase and hgcB, a ferredoxin-like protein. To date, all microbes possessing orthologs of these genes that have been tested are capable of methylating mercury; whereas, organisms lacking hgcA and hgcB are not. Also of interest is the observation that confirmed mercury-methylating microbes are all considered anaerobes although not members of a specific phylogenetic group. They are found scattered in the genomes of methanogens, Firmicutes, and Deltaproteobacteria. Methylation has not been demonstrated to provide protection of the microbes to mercury exposure. To determine the source of evolutionary pressure for acquisition and maintenance of these genes, we are seeking to understand whether there is a second function of the proteins. We are seeking evidence for the metabolic source(s) of the methyl group and for competing reactions. We have found that deletion of the metH gene encoding a tetrahydrofolate methyltransferase in Desulfovibrio sp. ND132 decreases the mercury methylation capacity by ca. 95%, consistent with an interpretation that this enzyme is involved in the pathway for the methyl group for HgcA. In addition, the corrinoid present in HgcA and the MetH of ND132 is strictly dependent on nicotinate nucleotide:5,6-dimethylbenzimidazole phosphoribosyltransferase encoded by the cobT gene, linking methionine biosynthesis with mercury methylation at a second level. Additional methyl transferases have not been found to be necessary for this function. While earlier evidence was provided for an involvement of the CO dehydrogenase/acetylCoA synthase, this enzyme is not universally present in methylating strains unlike the pathway for methionine synthesis.
