The excessive utilization of heavy metals through various industrial activity, agricultural activity and military operation has widely threaten the environment which is also threat to human health. Especially mercury is considered to be toxic and most hazardous metal on earth. Mercury is 6th most toxic element among the superabundance of 6 million substances, and mercury has an ability to get methylated to highly toxic organomercuria. Organomercurial compounds are very toxic to organisms as it leads to their bioaccumulation and enters in food chain.

The mercury reservoir is estimated to be 200 million tons in ocean. The dissolved Hg(II) concentration in water typically ranges from 0.03 to 2.0 sg/liter (i.e., up to 10-8 M) and, with this as backdrop, it is not surprising that bacteria have developed resistance mechanisms to detoxify both organomercurial and inorganic mercuric salts. In the biogeochemical cycle for mercury, bacteria recycle organomercurials back to nontoxic, volatile, elemental Hg(0). The elemental form of mercury (Hg0) is very volatile and is readily released into the atmosphere. An ionic form Hg(II) exists which maybe methylated to methylmercury (CH3Hg+) by bacteria. Methylmercury is the most toxic form of mercury and is bioaccumulated in fish and other aquatic animals. Human consumption of fish with elevated levels of health concerns. Mercury resistant bacteria have the ability to convert the methylmercury to the more volatile Hg0, thereby detoxifying methylmercury and removing mercury from the aquatic environment. This is carried out by a two-enzyme system that is encoded for within the mer operon. The mer B gene encodes for organomercury lyase, which converts CH3Hg+ to Hg(II). Mercuric reductase, encoded for by the mer A gene then converts Hg(II) to Hg0 which is released into the environment.

mer Operon

First reported bacterial resistance to inorganic and organic mercury compounds (HgR) in a clinical isolate of Staphylococcus aureus which will be also penicillin-resistantwas repoted by Moore
Microbes have developed resistance systems against toxic mercury compound to overcome the toxic environment. Among those resistance systems one of them is based on cluster of genes known as mer operon (operon found in microbes) for detoxification of mercuric ion (Hg2+) to (Hg+) by an enzymatic reduction reaction.
Several genes are involved in mer operon, that are merR/merD for detection, merP/merT/merC for transportation or mobilization, and finally merA/merB for enzymatic detoxification of inorganic and organic mercury compounds in bacteria. This operon is dysfunctional until it get in contact with mercurial compounds. Gene merR regulates the other genes of operon, which gets regulated only on exposure to mercuric ion (Hg2+).

Role of merA gene:
Mercury reductase enzyme (merA) including other genes get activated as the product of transcription of merR. merA, a flavin oxidoredutase, is fundamentally responsible for the reduction of highly toxic ionic Hg2+ into less toxic and volatile Hg0 in a NAD(P)H-dependent reaction. As a result volatile Hg0 is fluxed out from cytosolic region into outer periplasmic region. The amino terminal domain of merA is found to be homologous with small periplasmic mercury-binding protein merP which transfers Hg2+ to merT. The same mechanism by which merT transfers Hg2+ into cytosol is not clearly understood but it is predicted that a pair of cysteine residue is involved in the process.

Role of merB gene:

merB genes code for the enzyme organomercuryl lyase which is one of the most important enzyme for detoxification and bioremediation of the organomercurial compound. Phylogenetic analysis of various bacteria shows that merB genes codes for organomercury lyase, which is one of the unique enzymes whose homolog forms are not known. The enzyme organmercuryl lyase catalyses the protonolysis of carbon-mercury bound, thereby releasing less mobile and less toxic Hg2+ species which is further acted upon by merA enzyme for complete volatilization of organomercurial compounds. The merB gene is considered as an ancillary component of the mer operon. In most cases, merB gene will be found to be mapped immediately downstream of merA gene.

Types of mercury resistant mechanisms:
(1) Narrow spectrum: Only merA gene is present and resistance mechanism is limited to enzymatic detoxification of only inorganic mercury compound.

(2) Broad spectrum: It contains an extra gene merB in which tolerance is exhibited to organic as well as inorganic mercurial compounds by converting both forms of compounds to their volatile forms.

In general, narrow spectrum mercury-resistant operon (e.g., merRTPADE) confers resistance to only inorganic mercurial compounds, while the board spectrum mercury-resistant operon (e.g., merRTPAGBDE) confers resistant to both inorganic and organic mercurial compounds.

In Gram-negative bacteria MerG, a typical periplasmic protein is found to provide resistance against organomercurial in merB deficient bacterial strains. Therefore, presence of merA along with merG may show the effect of broad spectrum mercury detoxification in such bacterial strains. Various species of floras including, tobacco, Arabidopsis and chlorella have been biotechnologically modified to incorporated merA and merB genes that carried out detoxification of mercury-contaminated soil as a part of bioremediation. This also further supports the flexibility and adaptability of bacterial mer operon as an inter-species compatible genetic mechanism for tolerance against mercurial toxicity.

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An enthuiastic research scholar from India