We are just beginning to understand the metabolism of heavy metals and to use their metabolic functions in biotechnology, although heavy metals comprise the major part of the elements in the periodic table. Because they can form complex compounds, some heavy metal ions are essential trace elements, but, essential or not, most heavy metals are toxic at higher concentrations. This review describes the workings of known metal-resistance systems in microorganisms. After an account of the basic principles of homoeostasis for all heavy-metal ions, the transport of the 17 most important (heavy metal) elements is compared.

Microbial heavy-metal resistance

With global heavy metal contamination increasing, plants that can process heavy metals might provide efficient and ecologically sound approaches to sequestration and removal. Mercuric ion reductase, MerA, converts toxic Hg2+ to the less toxic, relatively inert metallic mercury (Hg0) The bacterial merA sequence is rich in CpG dinucleotides and has a highly skewed codon usage, both of which are particularly unfavorable to efficient expression in plants. We constructed a mutagenized merA sequence, merApe9, modifying the flanking region and 9% of the coding region and placing this sequence under control of plant regulatory elements. Transgenic Arabidopsis thaliana seeds expressing merApe9 germinated, and these seedlings grew, flowered, and set seed on medium containing HgCl2 concentrations of 25-100 microM (5-20 ppm), levels toxic to several controls. Transgenic merApe9 seedlings evolved considerable amounts of Hg0 relative to control plants. The rate of mercury evolution and the level of resistance were proportional to the steady-state mRNA level, confirming that resistance was due to expression of the MerApe9 enzyme. Plants and bacteria expressing merApe9 were also resistant to toxic levels of Au3+. These and other data suggest that there are potentially viable molecular genetic approaches to the phytoremediation of metal ion pollution.

https://www.pnas.org/content/pnas/93/8/3182.full.pdf

Mercuric ion reduction and resistance in transgenic Arabidopsis thaliana plants expressing a modified bacterial merA gene

Genetic engineering in the improvement of plants for phytoremediation of metal polluted soils.

This chapter concentrates on selected aspects of microbial mercury reduction and other mechanisms of mercuric ion resistance which are relevant to the interaction of metals and microorganisms in the environment. Mercury (Hg) is simple to refine—the ores are roasted in a current of air, and metallic mercury is condensed from the vapor. The simplicity of refining and the unusual properties of this metal probably account for the long and fascinating human relationship with mercury. Hg has been released into the lithosphere, atmosphere, and hydrosphere over millennia by geochemical processes, and it is therefore an important toxic element in the biosphere. The high affinity of mercury for thiol and imino nitrogen groups in proteins and the diverse cellular targets of mercuric ions preclude some of the main strategies used by microorganisms to avoid, eliminate, or detoxify other toxic metals. Using the criterion that a specific metal resistance is regulated by a specific discriminatory, regulatory component, some of the mechanisms discussed are simply tolerance mechanisms. Experimental and pilot scale microbial mercury reduction and volatilization systems have been developed for removal of Hg(II). Hgr is an underexploited model system for the study of the evolution of an environmental trait across different prokaryotic genera. Great progress has been made in one's understanding of the genetics and biochemistry of reductive Hgr.

Microbial Mercury Reduction

Multi-dose applying EDTA to decrease the potential groundwater contamination using rainbow pink (Dianthus chinensis) for enhanced phytoextraction

The gene merA coding for bacterial mercuric ion reductase was cloned under the control of the yeast promoter for alcohol dehydrogenase I in the yeast-Escherichia coli shuttle plasmid pADH040-2 and transformed into Saccharomyces cerevisiae AH22. The resulting transformant harbored stable copies of the merA-containing hybrid plasmid, displayed a fivefold increase in the MIC of mercuric chloride, and synthesized mercuric ion reductase activity. Images

Expression of bacterial mercuric ion reductase in Saccharomyces cerevisiae.

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Expression of bacterial mercuric ion reductase in Saccharomyces cerevisiae.