causes increased expression of metallothionein (mt1) as well as polyphosphate
kinase (ppk) genes. Metallothioneins are proteins which are membrane-bound and
bind with metals subsequently reducing them. This modified bacterium stores Hg
over 100 μM. Radiation-tolerant Deinococcus radiodurans has been transformed
through the mercuric ion reductase gene (merA) of E. coli BL308 host by Brim et al.
(2000). This mutant strain flourishes under both high radiation and mercury. They
have also volatilized inorganic Hg (Hg
2+ ) into elemental Hg (Hg
0 ). Dash and Das
(2015) have produced transgenic strain B. cereus BW- 03(pPW-05) in the same
manner which contains plasmid of a wild strain of B. thuringiensis PW-05
containing mer operon. Hg volatilization and Hg biosorption, the two processes of
mercury tolerance, are present in the resultant transgenic strain which can be
commonly employed for in situ elimination of mercury from the polluted
environments. To tolerate mercury, genetic alteration of heavy metal-tolerant bacterium C. metallidurans strain MSR33 was done (Rojas et al. 2011). This bacterium
can resist inorganic mercury as well as methyl mercury both with copper and
chromate. According to Chaturvedi and Archana (2014), two metal-binding peptides
were expressed in D. radiodurans R1, which is an attractive approach to develop
metal resistance. A synthetic gene (EC20) was created through overlap extension
which was expressed in DR1 under the native groESL promoter and codes for
phytochelatin analogue as well as cyanobacterial metallothionein (MT) gene,
smtA. This recombinant strain is 2.5-fold more tolerant to Cd
2+ and stored 1.21fold more Cd
2+ . Thus, integrating desirable genes inside compatible bacteria
provides improved strategies to remediate the environment.
12.10.2 Modification of Intrinsic Genes
There are several drawbacks for utilizing a genetically modified microorganism in
various environmental conditions. Some of the incorporated genes may be unstable
in different environments to perform the desired role (Dixit et al. 2015). In some
cases, under the presence of pollutants, the expression level of the foreign genes is
influenced so much that it has a negative impact on bioremediation applications
(Kiyono and Pan-Hou 1999). Therefore, as an alternative to these foreign genes, the
native genes may be attacked or modified. This method permits the microorganisms
growth in their natural surroundings besides performing the desired roles in
bioremediating metals. There are several investigations which focus on changing/
modifying the already existing group of genes of native microflora for bioremediation. Three cadmium-tolerant P. aeruginosa strains were isolated from industrial
sludge by Kermani et al. (2010). Upon exposure to the dyes acridine orange and
acriflavine, mutations in these strains could tolerate 7 mM of Cd
2+ . In E. coli,
cloning and overexpression of phytochelatin synthase of Schizosaccharomyces
pombe which genetically altered E. coli strain to store 25 times more Cd
2+ than
control strain (Kang et al. 2007). According to Wu et al. (2006), in the soil bacterium
P. putida 06909 cadmium, the binding ability was increased three times more
through overexpression of a synthetic phytochelatin EC20 which also doubled the
346
N. Srivastava
kinase (ppk) genes. Metallothioneins are proteins which are membrane-bound and
bind with metals subsequently reducing them. This modified bacterium stores Hg
over 100 μM. Radiation-tolerant Deinococcus radiodurans has been transformed
through the mercuric ion reductase gene (merA) of E. coli BL308 host by Brim et al.
(2000). This mutant strain flourishes under both high radiation and mercury. They
have also volatilized inorganic Hg (Hg
2+ ) into elemental Hg (Hg
0 ). Dash and Das
(2015) have produced transgenic strain B. cereus BW- 03(pPW-05) in the same
manner which contains plasmid of a wild strain of B. thuringiensis PW-05
containing mer operon. Hg volatilization and Hg biosorption, the two processes of
mercury tolerance, are present in the resultant transgenic strain which can be
commonly employed for in situ elimination of mercury from the polluted
environments. To tolerate mercury, genetic alteration of heavy metal-tolerant bacterium C. metallidurans strain MSR33 was done (Rojas et al. 2011). This bacterium
can resist inorganic mercury as well as methyl mercury both with copper and
chromate. According to Chaturvedi and Archana (2014), two metal-binding peptides
were expressed in D. radiodurans R1, which is an attractive approach to develop
metal resistance. A synthetic gene (EC20) was created through overlap extension
which was expressed in DR1 under the native groESL promoter and codes for
phytochelatin analogue as well as cyanobacterial metallothionein (MT) gene,
smtA. This recombinant strain is 2.5-fold more tolerant to Cd
2+ and stored 1.21fold more Cd
2+ . Thus, integrating desirable genes inside compatible bacteria
provides improved strategies to remediate the environment.
12.10.2 Modification of Intrinsic Genes
There are several drawbacks for utilizing a genetically modified microorganism in
various environmental conditions. Some of the incorporated genes may be unstable
in different environments to perform the desired role (Dixit et al. 2015). In some
cases, under the presence of pollutants, the expression level of the foreign genes is
influenced so much that it has a negative impact on bioremediation applications
(Kiyono and Pan-Hou 1999). Therefore, as an alternative to these foreign genes, the
native genes may be attacked or modified. This method permits the microorganisms
growth in their natural surroundings besides performing the desired roles in
bioremediating metals. There are several investigations which focus on changing/
modifying the already existing group of genes of native microflora for bioremediation. Three cadmium-tolerant P. aeruginosa strains were isolated from industrial
sludge by Kermani et al. (2010). Upon exposure to the dyes acridine orange and
acriflavine, mutations in these strains could tolerate 7 mM of Cd
2+ . In E. coli,
cloning and overexpression of phytochelatin synthase of Schizosaccharomyces
pombe which genetically altered E. coli strain to store 25 times more Cd
2+ than
control strain (Kang et al. 2007). According to Wu et al. (2006), in the soil bacterium
P. putida 06909 cadmium, the binding ability was increased three times more
through overexpression of a synthetic phytochelatin EC20 which also doubled the
346
N. Srivastava