B. pumilus S3 (De et al. 2008), while immobilization of lead into pyromorphite
which is an insoluble lead phosphate mineral makes phosphate-solubilizing bacterium E. cloacae lead tolerant (Park et al. 2011). This lead reclamation process is an
efficient, environmental friendly lead bioremediation strategy. Another approach for
detoxification is the induction of siderophores under lead stress. There is a production of siderophores like pyochelin and pyoverdine in lead-resistant P. aeruginosa
strain 4EA through activation by lead (Naik and Dubey 2011). Thus, genetic
modifications providing lead resistance followed by several biotransformation
procedures for remediating lead may be utilized for microbial remediation of lead.
12.7.4 Mercury
Mercury is one of the highly dangerous elements in the universe and has severe
health consequences compared to other harmful metal contaminants. It becomes
concentrated in sediments, soils, atmosphere and water. The most probable source of
mercury ingestion is fish eating. Mercury also gets an entry in the environment as
industrial wastes. Two separate operons provide mercury resistance in bacteria. One
of them is a narrow-spectrum mer operon, and the other one is the broadspectrum
mer operon (Silver and Phung 2013). Transposons, Tn5037 (Kalyaeva et al. 2001)
and Tn5070 (Mindlin et al. 2001) possess most simple mer operons, while the most
complicated mer operon is found present at Tn5718 (Schneiker et al. 2001). Genes
merR, merT, merC, merF, merP and merD are present on the narrow-spectrum mer
operon (Dash and Das 2012). This operon is induced by inorganic mercury (Hg
2+ )
and gives tolerance just for inorganic mercury salts. merR behaves like the positive
transcriptional regulator of the operon and is transcribed independently. Irrespective
of the existence of extracellular mercury or lack of intracellular mercury, it combines
with the promoter/operator portion of the operon for controlling the induction of the
functional genes positively as well as negatively. MerP is the outermost protein of
the operon and found situated in the periplasmic space. It has a β-α-β-β-α-β fold and
has 72 amino acids. The two α helices are overlaying the four-strand antiparallel β
sheet (Eriksson and Sahlman 1993). The mercury-binding site has the GMTCAAC
consensus sequence. MerP functions as a scavenger of inorganic mercury ions and
brings them on the MerT protein (Hamlett et al. 1992). MerT is coded through the
transposon Tn501 and is a 116-amino acid protein, which accepts the inorganic
mercury from MerP on the plasma membrane and transfers it in the cell by combining with cysteine residues over its transmembrane helices. Mercuric resistance will
be lost by eliminating or mutating these residues. The precise method of MerC is
unclear. But, it was observed that MerC has a role in mercury transportation and
accumulation (Kiyono et al. 2013). Induction of merC in Arabidopsis thaliana and
Nicotiana tabacum has doubled their capacity of storing mercury (Sasaki et al.
2006). On the contrary, MerF is a 8.7-kDa protein which possesses two transmembrane helices. It also behaves like a broad-spectrum mercury transporter. Mercuric
ion reductase is coded by a 1600-nucleotide-long merA gene (Moore et al. 1990).
This dimeric enzyme’s role is to decrease the level of Hg
2+ in the cell for producing
338
N. Srivastava
which is an insoluble lead phosphate mineral makes phosphate-solubilizing bacterium E. cloacae lead tolerant (Park et al. 2011). This lead reclamation process is an
efficient, environmental friendly lead bioremediation strategy. Another approach for
detoxification is the induction of siderophores under lead stress. There is a production of siderophores like pyochelin and pyoverdine in lead-resistant P. aeruginosa
strain 4EA through activation by lead (Naik and Dubey 2011). Thus, genetic
modifications providing lead resistance followed by several biotransformation
procedures for remediating lead may be utilized for microbial remediation of lead.
12.7.4 Mercury
Mercury is one of the highly dangerous elements in the universe and has severe
health consequences compared to other harmful metal contaminants. It becomes
concentrated in sediments, soils, atmosphere and water. The most probable source of
mercury ingestion is fish eating. Mercury also gets an entry in the environment as
industrial wastes. Two separate operons provide mercury resistance in bacteria. One
of them is a narrow-spectrum mer operon, and the other one is the broadspectrum
mer operon (Silver and Phung 2013). Transposons, Tn5037 (Kalyaeva et al. 2001)
and Tn5070 (Mindlin et al. 2001) possess most simple mer operons, while the most
complicated mer operon is found present at Tn5718 (Schneiker et al. 2001). Genes
merR, merT, merC, merF, merP and merD are present on the narrow-spectrum mer
operon (Dash and Das 2012). This operon is induced by inorganic mercury (Hg
2+ )
and gives tolerance just for inorganic mercury salts. merR behaves like the positive
transcriptional regulator of the operon and is transcribed independently. Irrespective
of the existence of extracellular mercury or lack of intracellular mercury, it combines
with the promoter/operator portion of the operon for controlling the induction of the
functional genes positively as well as negatively. MerP is the outermost protein of
the operon and found situated in the periplasmic space. It has a β-α-β-β-α-β fold and
has 72 amino acids. The two α helices are overlaying the four-strand antiparallel β
sheet (Eriksson and Sahlman 1993). The mercury-binding site has the GMTCAAC
consensus sequence. MerP functions as a scavenger of inorganic mercury ions and
brings them on the MerT protein (Hamlett et al. 1992). MerT is coded through the
transposon Tn501 and is a 116-amino acid protein, which accepts the inorganic
mercury from MerP on the plasma membrane and transfers it in the cell by combining with cysteine residues over its transmembrane helices. Mercuric resistance will
be lost by eliminating or mutating these residues. The precise method of MerC is
unclear. But, it was observed that MerC has a role in mercury transportation and
accumulation (Kiyono et al. 2013). Induction of merC in Arabidopsis thaliana and
Nicotiana tabacum has doubled their capacity of storing mercury (Sasaki et al.
2006). On the contrary, MerF is a 8.7-kDa protein which possesses two transmembrane helices. It also behaves like a broad-spectrum mercury transporter. Mercuric
ion reductase is coded by a 1600-nucleotide-long merA gene (Moore et al. 1990).
This dimeric enzyme’s role is to decrease the level of Hg
2+ in the cell for producing
338
N. Srivastava