Difference between revisions of "Part:BBa K1420000"

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Mer operon, biological system found to detoxify organic and inorganic forms of mercury

Function and Components of the mer operon

The mer operon is a set of genes that function in synchrony to convey mercury resistance to bacterial cells. For our purposes the operon can be used to bioremediate methylmercury by converting it to its less toxic ionic form, Hg(II), and then into volatile Hg(0). While variations in the operon exist, with not all genes being present in all organisms and some having extra genes, the operon we have focused on contains five genes: merR, merT, merP, merA and merB.

merR, located upstream of the rest of the mer operon mercury resistance genes, serves to regulate the mer operon by activating transcription in the presence of Hg(II) and acting as a weak repressor in the absence of Hg(II). The effector binding region of MerR family proteins can vary allowing for great diversity in MerR-like promoters that can respond to a variety of heavy metals as well as antibiotics and oxidative stress. This variable nature of MerR family proteins makes them a valuable tool for various heavy metal detection and bioremediation. Once transcription is activated, the mer T and merP genes encode for proteins that work collectively to transport Hg(II) species into the cell. merP encodes the periplasmic transport protein, MerP, which binds a single Hg(II) ion at two conserved cysteine residues that define its metal binding motif. The MerP cysteine residues take up a Hg(II) ion and remove any attached ligands before passing the ion to the MerT transmembrane protein. merT on the other hand encodes a transmembrane mercuric binding enzyme, MerT, which transports Hg(II) species from the periplasm through the membrane. Hg(II) is transferred from the periplasmic cysteine pair on the first transmembrane helix to the cytoplasmic loop cysteine, where it is finally transferred to a cysteine pair at the N-terminus of the protein. Once the Hg(II) species are in the cytoplasm, the merB gene, which is often found immediately downstream of merA, is essential for the detoxification and bioremediation of organic toxic mercury compounds in congruence with merA. The MerB enzyme is a lyase that catalyzes the breaking of carbon-mercury bonds through protonolysis of toxic mercury compounds, such as methylmercury. This produces the less toxic and less mobile Hg²⁺ which is then completely volatilized to Hg⁰. Lastly, enzyme MerA catalyzes the reduction of the mercuric ion, Hg⁰, to the relative inert, volatile monoatomic mercury in a NADPH dependent reaction. (For more information on the each individual gene of the mer operon, see the following parts pages: BBa_K1420001 for merA, BBa_K1420002 for merB, BBa_K1420003 for merP, BBa_K1420004 for merR, and BBa_K1420005 for merT.)

Construction of the pBBRBB::mer plasmid

Figure 1.

Sequence and Features