Team:BIOSINT Mexico/Bioaccumulation

Mercury Bioaccumulation

Description

Detection

Heavy metal-mediated toxicity has always been one of the greatest obstacles against survival of microorganisms. Bacteria, however, have evolved a great spectrum of mechanisms to deal with such impediment. The genetic system “mer operon” is the only bacterial metal resistance system with high yield transformation of its toxic target into volatile non-toxic forms. Basically, protein products of mer genes efficiently utilize the high affinity of Hg­2+ towards cysteine residues in proteins for their enzymatic degradation and transportation.(Mathema VB et al, 2011) Various genes are involved in mer operon, including MerR/MerD for detection, MerP/MerT/MerC/MerE for transportation or mobilization and MerB and MerA for enzymatic detoxification of both organic and inorganic mercury compounds in bacteria. These genes are governed by MerR, which gets activated during Hg2+ exposure. (Mathema VB et al, 2011)

Transportation mechanism

The mechanism of transport of Hg2+ and CH­3Hg across the bacterial membrane is mediated by MerC, MerE, MerT and MerP. Even though among the four transporters, MerC showed highest potential for Hg2+ transport across the bacterial membrane (Sone Y et al, 2013), for broader purposes of this work, MerE will be used to evaluate its effect on bioaccumulation of methylmercury in A. tumefaciens.

In addition MerE is able to transport methylmercury and ions of mercury ions across bacterial cytoplasmatic membranes. When arabidopsis thaliana works with the gene MerE it creates a resistance to methylmercury and to the mercury ions, demonstrating the MerE promoted the transport and accumulation of this two elements which facilitates the phytoremediation of methylmercury (Sone Y. et al, 2013).

MerE causes an hypersensitivity to CH3Hg, some experiments taken by Yuka Sone showed that in bacteria, the presence of CH3Hg was threefold as much as than cells without these gene (Sone Y. et al, 2010). and proved that MerE “maybe a universal system in Gram-negative bacteria since the aminoacid sequence of MerE from Tn21 show a high level of homology (73-78%)” (Sone Y. et al, 2010).

Reduction

MerB gene codes for organomercurial lyase, that catalyzes the protonolysis of the carbon-mercury bond.(Bizily S.P et al, 1999) The products of this reaction are a less toxic inorganic species Hg2+ and a reduced carbon compound.

In presence of MerB:

R-CH2-Hg++H+ R-CH3+Hg2+

“The kinetics of the MerB-catalyzed reaction could be constrained by the rate of diffusion of organomercurial substrates from cellular membrane systems to sites of catalysis or by the rate of diffusion of the product, Hg2+, away from the enzyme” (Bizily S.P et al, 1999).

MerB gives resistance to organomercurial lyase crating that plants efficiently protonolyze organic mercury producing a more tolerable mercury species (Bizily S.P et al, 1999) .

We know that Hg2+ is generated by MerB within the citoplasm in the presence of cellular proteins, it remains paradoxical that plants with MerE tolerate more organic mercury than wild-type plants (Bizily S.P et al, 1999). The reason of that is because, the lipid solubility of organic mercury gives access to mitochondria and chloroplasts where it may affect essential oxidative and photosynthetic electron transport chains. It is also probable that cytoplasmic chelators dins and sequester Hg2+ (Bizily S.P et al, 1999) in preference of organomercurials.

References

Bizily S., et al. (1999). Phytoremediation of methylmercury pollution: MerB expression in Arabidopsis Thaliana confers resistance to organomercurials. Proc. Natl. Acad. Vol. 96, pp. 6806-6813, June1999.

Sone Y., et al. (2010). Roles played by MerE and MerT in the transport of inorganic and organic mercury compounds in Gram-negative Bacteria. Journal of Health Science, 56(1) 123-127 2010.

Sone Y., et al. (2013). Increase methylmercury accumulation in Arabidopsis thaliana expressing bacterial broad-spectrum mercury transporter MerE. AMB Express 2013 3:52.

[4] Barkay, T., Miller, S. and Summers, A. (2003). Bacterial mercury resistance from atoms to ecosystems. FEMS microbiology reviews, 27(2-3), pp.355--384.