Unexpected Effects of Gene Deletion on Interactions of Mercury with the Methylation-Deficient Mutant ΔhgcAB

The hgcA and hgcB gene pair is essential for mercury (Hg) methylation by certain anaerobic bacteria, but little is known about how deletion of hgcAB affects the cell surface interactions and intracellular uptake of Hg. Here, we compare ΔhgcAB mutants with the wild-type (WT) strains of both Geobacter sulfurreducens PCA and Desulfovibrio desulfuricans ND132 and observe differences in Hg redox transformations, adsorption, and uptake in laboratory incubation studies. In both strains, deletion of hgcAB increased the rate of reduction of Hg(II) but decreased the rate of oxidation of Hg(0) under anaerobic conditions. The measured cellular thiol content in ΔhgcAB mutants was lower than that in the WT, accounting for the decreased rates of adsorption and uptake of Hg. Despite the lack of methylation activity, uptake of Hg by the ΔhgcAB mutant continued, albeit at a rate slower than that of the WT. These findings demonstrate that deletion of the hgcAB gene pair not only eliminates Hg methylation but also alters cell physiology, resulting in changes to Hg redox reactions, sorption, and uptake by cells.