with the help of Hg reductase (Barkay et al. 2003). The narrow Hg tolerant mer
operon breaks down mercury in the sequence of three successive stages, which are
transfer of Hg
2+ into the cell, NADPH-dependent enzymatic transformation of the
ionic mercury into less toxic elementary mercury (Hg
0 ) and at last the control of
those functional genes which are involved in the transport and transformation (Singh
et al. 2011). This operon possesses several genes which are functionally different
like merA and merB involved in the reduction, merT and merP working like
transporters, mer R and merD used in the regulation and newly discovered merE
and merH used for membrane transport (Ruiz and Daniell 2009; Kiyono et al. 2009;
Schue et al. 2009). Genetically modified E. coli possess simultaneously merT-merP
and MT genes which have the potential to eliminate Hg
2+ efficiently from electrolytic wastewater (Deng and Wilson 2001). Deinococcus radiodurans, the highst
radiation-tolerant bacterium, was altered through the expression of merA gene from
E. coli BL308. The resultant recombinant was able to grow in radiations and high
levels of ionic mercury. It has efficiently reduced Hg ions to less harmful volatile
elemental mercury (Brim et al. 2006). Metal-binding peptides phytochelatins as well
as metallothioneins increase the ability to bind metals in microorganisms (Bae et al.
2001). Cloning of the genes of these proteins has been done in E. coli from plants
and fungi. Recombinant bacteria, having increased bioaccumulation ability and
more affinity for chosen metal, select the metal ions for storing from multicomponent contaminants (Sauge-Merle et al. 2003). For bioaccumulating Cd
2+ from the
heavy metal contaminated place, Deng et al. (2007) developed a genetically
modified E. coli JM109. At the same time, by expressing fusion proteins for
phytochelatin (PC), metal-binding peptide (EC20), and RsaA-6His, respectively,
several genetically modified bacteria like Mesorhizobium huakuii, P. putida and
Caulobacter crescentus have been developed for the storage of Cd
2+ (Sriprang et al.
2003; Wu et al. 2010; Patel et al. 2010). Arsenic (As) is highly dangerous for animal
health as it is stored in the edible portion of plants, soil and polluted water. From
here, this inorganic compound enters the food chain. Because of this, to urgently
eliminate As from the polluted location, arsenite S-adenosylmethionine
methyltransferase gene (arsM) was isolated from Rhodopseudomonas palustris
and cloned in E. coli (Qin et al. 2006), which methylated inorganic As into less
harmful and volatile trimethylarsine (TMA). Recombinant bacteria modified through
arsM gene eliminate As from the polluted soil perfectly by volatilizing As (Liu et al.
2011). These recombinant bacteria stored more As than control strain. Expression of
another metalloregulatory protein ArsR in E. coli provided more affinity to eliminate
As from the polluted sites (Kostal et al. 2004). In several organisms, more affinity for
membrane transport of Ni
2+ is provided because of the metallothionein protein
(MT), glutathione S-transferase fusion protein (GST-MT) and nixA- encoded membrane transport protein (Singh et al. 2011). It is thought that higher levels of Ni
2+
could be stored in the engineered bacteria due to enhanced expression of these
transport proteins (Kumar et al. 2013).
348
N. Srivastava
operon breaks down mercury in the sequence of three successive stages, which are
transfer of Hg
2+ into the cell, NADPH-dependent enzymatic transformation of the
ionic mercury into less toxic elementary mercury (Hg
0 ) and at last the control of
those functional genes which are involved in the transport and transformation (Singh
et al. 2011). This operon possesses several genes which are functionally different
like merA and merB involved in the reduction, merT and merP working like
transporters, mer R and merD used in the regulation and newly discovered merE
and merH used for membrane transport (Ruiz and Daniell 2009; Kiyono et al. 2009;
Schue et al. 2009). Genetically modified E. coli possess simultaneously merT-merP
and MT genes which have the potential to eliminate Hg
2+ efficiently from electrolytic wastewater (Deng and Wilson 2001). Deinococcus radiodurans, the highst
radiation-tolerant bacterium, was altered through the expression of merA gene from
E. coli BL308. The resultant recombinant was able to grow in radiations and high
levels of ionic mercury. It has efficiently reduced Hg ions to less harmful volatile
elemental mercury (Brim et al. 2006). Metal-binding peptides phytochelatins as well
as metallothioneins increase the ability to bind metals in microorganisms (Bae et al.
2001). Cloning of the genes of these proteins has been done in E. coli from plants
and fungi. Recombinant bacteria, having increased bioaccumulation ability and
more affinity for chosen metal, select the metal ions for storing from multicomponent contaminants (Sauge-Merle et al. 2003). For bioaccumulating Cd
2+ from the
heavy metal contaminated place, Deng et al. (2007) developed a genetically
modified E. coli JM109. At the same time, by expressing fusion proteins for
phytochelatin (PC), metal-binding peptide (EC20), and RsaA-6His, respectively,
several genetically modified bacteria like Mesorhizobium huakuii, P. putida and
Caulobacter crescentus have been developed for the storage of Cd
2+ (Sriprang et al.
2003; Wu et al. 2010; Patel et al. 2010). Arsenic (As) is highly dangerous for animal
health as it is stored in the edible portion of plants, soil and polluted water. From
here, this inorganic compound enters the food chain. Because of this, to urgently
eliminate As from the polluted location, arsenite S-adenosylmethionine
methyltransferase gene (arsM) was isolated from Rhodopseudomonas palustris
and cloned in E. coli (Qin et al. 2006), which methylated inorganic As into less
harmful and volatile trimethylarsine (TMA). Recombinant bacteria modified through
arsM gene eliminate As from the polluted soil perfectly by volatilizing As (Liu et al.
2011). These recombinant bacteria stored more As than control strain. Expression of
another metalloregulatory protein ArsR in E. coli provided more affinity to eliminate
As from the polluted sites (Kostal et al. 2004). In several organisms, more affinity for
membrane transport of Ni
2+ is provided because of the metallothionein protein
(MT), glutathione S-transferase fusion protein (GST-MT) and nixA- encoded membrane transport protein (Singh et al. 2011). It is thought that higher levels of Ni
2+
could be stored in the engineered bacteria due to enhanced expression of these
transport proteins (Kumar et al. 2013).
348
N. Srivastava