FIGURE 3.

Bacterial resistance to mercury. The mer operon consists of a cluster up to 8 genes merTPCAGBDE. Upon the entry of the inorganic form of Hg (Hg²⁺) via non-specific porin proteins, the first protein to bind it is MerP, a small periplasmic chaperone. MerP transports Hg²⁺ to MerT, MerC or MerF, which are inner membranous mercuric ions-binding proteins that in turn transport Hg²⁺ to the cytoplasm. merT is the most prevalent gene within the mer operon as compared to merC and merF. In the cytoplasm, MerA detoxifies Hg²⁺ ions through reduction-catalyzed volatilization process to a non-toxic elemental form Hg⁰. This form is volatile at room temperature; it diffuses outside the membranes allowing the bacterial cell to escape Hg toxicity (Nies, 1999; Silver and Phung le, 2005; Boyd and Barkay, 2012; Hobman and Crossman, 2014). MerE is an inner membranous protein of unknown function (Silver and Phung le, 2005). In Gram-negative bacteria, the mer operon is regulated by MerR, which is in turn activated by increased Hg²⁺ levels in the cytoplasm. This induces the expression of the whole merTPCAGBDE operon (Boyd and Barkay, 2012; Hobman and Crossman, 2014). Resistance to the organic form of Hg (CH₃-Hg⁺) is achieved by merB, which encodes an Organomercurcial Lyase (MerB) located in the cytoplasm. When CH₃-Hg⁺ enter the cytoplasm via non-specific porin proteins, MerB cleaves the Mercury-Carbon bond and releases Hg²⁺ in the cytoplasm. At this point, Hg²⁺ is reduced to Hg⁰ that diffuse outside the bacterial cell (Nies, 1999; Boyd and Barkay, 2012; Hobman and Crossman, 2014; Pal et al., 2017). Adapted and modified with permission from Silver and Phung le (2005), Boyd and Barkay (2012) and Pal et al. (2017).