merB

MerB is a gene encoding an organomercurial lyase found in many bacteria as part of the mercury resistance (mer) operon. It is commonly located alongside merA, which encodes mercuric reductase, as well as periplasmic binding and transport proteins such as merP and merT and regulatory elements like merR. MerB catalyzes the cleavage of carbon–mercury bonds in organic mercury compounds, such as methylmercury and phenylmercury, releasing inorganic mercury in the form of Hg2+ and leaving behind a corresponding hydrocarbon fragment.

In typical mer operons, MerA reduces the liberated Hg2+ to volatile elemental mercury (Hg0), enabling detoxification and volatilization. MerB activity thus serves as the first step in a two-enzyme pathway that converts toxic organomercury compounds into less harmful forms. The transport proteins MerP and MerT facilitate movement of mercury species within the cell, delivering Hg2+ to MerA for reduction.

Genetic organization and regulation of merB vary, but many operons are located on plasmids or chromosomes and are inducible by the presence of mercury compounds. The MerR transcriptional regulator senses Hg2+ or organomercury and activates expression of the operon; in some systems MerD acts as a negative regulator to modulate expression levels.

Distribution and significance: MerB is widespread among Gram-negative bacteria and contributes to microbial mercury resistance and cycling in the environment. It is a focal point of bioremediation research, where organisms or engineered systems exploit MerB, in conjunction with MerA, to detoxify organomercury-contaminated sites and reduce ecological mercury burdens.