Microbial Diversity of Mer Operon Genes and Their Potential Rules in Mercury Bioremediation and Resistance

Mercury is a toxic metal that is present in small amounts in the environment, but its level is rising steadily, due to different human activities, such as industrialization. It can reach humans through the food chain, amalgam fillings, and other sources, causing different neurological disorders, memory loss, vision impairment, and may even lead to death; making its detoxification an urgent task.

Various physical and chemical mercury remediation techniques are available, which generally aim at: (i) reducing its mobility or solubility; (ii) causing its vaporization or condensation; (iii) its separation from contaminated soils. Biological remediation techniques, commonly known as bioremediation, are also another possible alternative, which is considered as cheaper than the conventional means and can be accomplished using either (i) organisms harboring the mer operon genes (merB, merA, merR, merP, merT, merD, merF, merC, merE, merH and merG), or (ii) plants expressing metal-binding proteins. Recently, different mer determinants have been genetically engineered into several organisms, including bacteria and plants, to aid in detoxification of both ionic and organic forms of mercury.

Bacteria that are resistant to mercury compounds have at least a mercuric reductase enzyme (MerA) that reduces Hg⁺² to volatile Hg⁰, a membrane-bound protein (MerT) for Hg⁺² uptake and an additional enzyme, MerB, that degrades organomercurials by protonolysis. Presence of both merA and merB genes confer broad-spectrum mercury resistance. However, merA alone confers narrow spectrum inorganic mercury resistance.

To conclude, this review discusses the importance of mercury-resistance genes in mercury bioremediation. Functional analysis of mer operon genes and the recent advances in genetic engineering techniques could provide the most environmental friendly, safe, effective and fantastic solution to overcome mercuric toxicity.