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). As there are various genes involved in mer operon that constitute the different parts of the mercurial environment detoxification mechanism, we selected from these pull of genes those that could be better suited for our chassis.

Transportation mechanism

The mechanism of transport of Hg2+ and CH­3Hg inherently 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 was picked to evaluate its effect on bioaccumulation of methylmercury in A. thaliana.

In addition MerE is able to transport methylmercury and ions of mercury ions across bacterial cytoplasmic membranes. When arabidopsis thaliana works with the gene MerE it has been demonstrated to gain resistance to methylmercury and to the mercury ions, promoting the transport and accumulation of this two elements and eventually facilitating methyl mercury phytoremediation (Sone Y. et al, 2013).

Reduction

MerB gene codes for organomercurial lyase, that catalyzes the protonolysis of the carbon-mercury bond.(Bizily S.P et al, 1999) Thus the products of this reaction are a less toxic inorganic Hg2+ species 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 allowing that plants efficiently protonolyze organic mercury producing a more tolerable mercury species (Bizily S.P et al, 1999) .

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 this situation is that 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. Sustaining, in this case, the function of our module by naturally selecting methyl mercury over mercury´s ioinc form and inducing our system more continously.

Modeling

Results

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.