The Genetic Basis for Bacterial Mercury Methylation

Mercury Methylating Microbes Mercury (Hg) most commonly becomes bioavailable and enters the food web as the organic form methylmercury, where it induces acute toxicity effects that can be magnified up the food chain. But most natural and anthropogenic Hg exists as inorganic Hg2+ and is only transformed into methylmercury by anaerobic microorganisms—typically sulfur-reducing bacteria. Using comparative genomics, Parks et al. (p. 1332, published online 7 February; see the Perspective by Poulain and Barkay) identified two genes that encode a corrinoid and iron-sulfur proteins in six known Hg-methylating bacteria but were absent in nonmethylating bacteria. In two distantly related model Hg-methylating bacteria, deletion of either gene—or both genes simultaneously—reduced the ability for the bacteria to produce methylmercury but did not impair cellular growth. The presence of this two-gene cluster in several other bacterial and lineages for which genome sequences are available suggests the ability to produce methylmercury may be more broadly distributed in the microbial world than previously recognized. A two-gene cluster encodes proteins required for the production of the neurotoxin methylmercury in bacteria. [Also see Perspective by Poulain and Barkay] Methylmercury is a potent neurotoxin produced in natural environments from inorganic mercury by anaerobic bacteria. However, until now the genes and proteins involved have remained unidentified. Here, we report a two-gene cluster, hgcA and hgcB, required for mercury methylation by Desulfovibrio desulfuricans ND132 and Geobacter sulfurreducens PCA. In either bacterium, deletion of hgcA, hgcB, or both genes abolishes mercury methylation. The genes encode a putative corrinoid protein, HgcA, and a 2[4Fe-4S] ferredoxin, HgcB, consistent with roles as a methyl carrier and an electron donor required for corrinoid cofactor reduction, respectively. Among bacteria and archaea with sequenced genomes, gene orthologs are present in confirmed methylators but absent in nonmethylators, suggesting a common mercury methylation pathway in all methylating bacteria and archaea sequenced to date.