volatile Hg
0 for easily discharging it from the cell. MerA protein is a flavoprotein,
which needs NADPH for donating electrons to carry out the reaction. MerD
produces a secondary regulator protein which combines weakly with the same
promoter/operator region as MerR and controls the operon negatively (Nascimento
and Chartone-Souza 2003). merH is recognized as a mercuric ion transporter (Schue
et al. 2009) which is situated upstream of the merA gene and transports Hg
2+ ions
through cysteine residues and induced simultaneously with merA itself. merH is
assumed to work like a metal-trafficking protein to merR, which subsequently
induces the mer operon to volatilize Hg
2+ through merA (Schelert et al. 2013).
merI is also recognized just downstream to merA gene (Schelert et al. 2006).
Although its exact role is still not known, a wide variety of patterns and order of
mer operon can be investigated further for bioremediation application (Rebello et al.
2013). The broad-spectrum mer operons have similarity in their genes with the
narrow-spectrum operons. Besides the already existing genes, some extra genes
such as merE, merG and merB are also found. The broad-spectrum operons defend
against organic mercury (Barkay et al. 2003). These compounds are very harmful
because they can go effortlessly inside the cell without the help of transporter
molecules. Organic mercury (R-Hg) diffuses passively inside the cell or is moved
inside through MerE or MerG. merE was primarily found in the transposon Tn21
and can assist in the translocation of methyl mercury (CH 3 Hg
+ ) as well as inorganic
mercury (Boyd and Barkay 2012). Tolerance to phenylmercury is because of MerG
which is a 20 kD protein produced by a 654-bp gene (Kiyono and Pan-Hou 1999). It
was found to be situated in between merA and merB on the operon and give
protection to the cell from phenylmercury through prohibiting its entry inside the
cell (Schneiker et al. 2001). Another enzymatic molecule of the mer operon merB
encodes the enzyme organmercurial lyase. MerB protein is catalyzing the
protonolysis of the carbon–mercury bond which produces ionic mercury as well as
a reduced hydrocarbon. The mercuric ion reductase reduces the ionic mercury to the
elemental form Hg
0 (Dash and Das 2012) which is volatilized out of the bacterial cell
because of its elevated vapour pressure. Microbes bioremediate mercury through
several enzymatic transformations such as reduction of Hg
2+ to Hg
0 , organomercurial compound breakdown, Hg
2+ methylation and oxidation of Hg
0 into Hg
2+ . A
transgenic bacterium B. cereus BW- 03(pPW-05) was constructed through the
transformation of the plasmid containing mer operon of marine bacterium
B. thuringiensis PW-05 found in the Bay of Bengal (Dash et al. 2014) into a novel
marine bacterium B. cereus BW-03 by Dash and Das (2015) having the capacity for
biosorption of mercury to resist mercury. It has eliminated (>99%) mercury supplement in laboratory condition through parallel volatilization (>53%) as well as
biosorption (>40%). Many mercury-tolerant mer gene-containing bacterial isolates
were separated which could volatilize and reduce Hg
2+ to Hg
0 (De et al. 2008).
Proteins of the mer operon reduce mercury as well as breakdown organomercurial
compounds. Additionally, the electrochemical potential of Hg
2+ /Hg
0 at pH 7 is
+430 mV which shows that Hg2
+ can be reduced to the elemental form by the living
cell which is harmless for bacteria. As the melting point/boiling point of mercury is
low (À39/357
C), metallic mercury can diffuse passively from the cell and can be
12 Remediation of Heavy Metals Through Genetically Engineered Microorganism
339
0 for easily discharging it from the cell. MerA protein is a flavoprotein,
which needs NADPH for donating electrons to carry out the reaction. MerD
produces a secondary regulator protein which combines weakly with the same
promoter/operator region as MerR and controls the operon negatively (Nascimento
and Chartone-Souza 2003). merH is recognized as a mercuric ion transporter (Schue
et al. 2009) which is situated upstream of the merA gene and transports Hg
2+ ions
through cysteine residues and induced simultaneously with merA itself. merH is
assumed to work like a metal-trafficking protein to merR, which subsequently
induces the mer operon to volatilize Hg
2+ through merA (Schelert et al. 2013).
merI is also recognized just downstream to merA gene (Schelert et al. 2006).
Although its exact role is still not known, a wide variety of patterns and order of
mer operon can be investigated further for bioremediation application (Rebello et al.
2013). The broad-spectrum mer operons have similarity in their genes with the
narrow-spectrum operons. Besides the already existing genes, some extra genes
such as merE, merG and merB are also found. The broad-spectrum operons defend
against organic mercury (Barkay et al. 2003). These compounds are very harmful
because they can go effortlessly inside the cell without the help of transporter
molecules. Organic mercury (R-Hg) diffuses passively inside the cell or is moved
inside through MerE or MerG. merE was primarily found in the transposon Tn21
and can assist in the translocation of methyl mercury (CH 3 Hg
+ ) as well as inorganic
mercury (Boyd and Barkay 2012). Tolerance to phenylmercury is because of MerG
which is a 20 kD protein produced by a 654-bp gene (Kiyono and Pan-Hou 1999). It
was found to be situated in between merA and merB on the operon and give
protection to the cell from phenylmercury through prohibiting its entry inside the
cell (Schneiker et al. 2001). Another enzymatic molecule of the mer operon merB
encodes the enzyme organmercurial lyase. MerB protein is catalyzing the
protonolysis of the carbon–mercury bond which produces ionic mercury as well as
a reduced hydrocarbon. The mercuric ion reductase reduces the ionic mercury to the
elemental form Hg
0 (Dash and Das 2012) which is volatilized out of the bacterial cell
because of its elevated vapour pressure. Microbes bioremediate mercury through
several enzymatic transformations such as reduction of Hg
2+ to Hg
0 , organomercurial compound breakdown, Hg
2+ methylation and oxidation of Hg
0 into Hg
2+ . A
transgenic bacterium B. cereus BW- 03(pPW-05) was constructed through the
transformation of the plasmid containing mer operon of marine bacterium
B. thuringiensis PW-05 found in the Bay of Bengal (Dash et al. 2014) into a novel
marine bacterium B. cereus BW-03 by Dash and Das (2015) having the capacity for
biosorption of mercury to resist mercury. It has eliminated (>99%) mercury supplement in laboratory condition through parallel volatilization (>53%) as well as
biosorption (>40%). Many mercury-tolerant mer gene-containing bacterial isolates
were separated which could volatilize and reduce Hg
2+ to Hg
0 (De et al. 2008).
Proteins of the mer operon reduce mercury as well as breakdown organomercurial
compounds. Additionally, the electrochemical potential of Hg
2+ /Hg
0 at pH 7 is
+430 mV which shows that Hg2
+ can be reduced to the elemental form by the living
cell which is harmless for bacteria. As the melting point/boiling point of mercury is
low (À39/357
C), metallic mercury can diffuse passively from the cell and can be
12 Remediation of Heavy Metals Through Genetically Engineered Microorganism
339