Mercury resistance system

Some metals can be converted to a less toxic form through enzyme detoxification. The most well-described example of this mechanism is the mercury resistance system, which occurs in S. aureus,43 Bacillus sp.,44 E. coli,45 Streptomyces lividans,46 and Thiobacillus ferrooxidans 47 The mer operon in these bacteria includes two different metal resistance mechanisms.48 MerA employs an enzyme detoxification approach as it encodes a mercury reductase, which converts the divalent mercury cation into elemental mercury 49 Elemental mercury is more stable and less toxic than the divalent cation. Other genes in the operon encode membrane proteins that are involved in the active transport of elemental mercury out of the cell.50 52... 

Resistance to the toxic effects of mercury is quite commonly found in bacteria (Silver and Walderhaug 1992 Misra 1992). Since most bacteria are rarely exposed to toxic levels of mercury, the resistance mechanism is inducible and is frequently found on plasmids and/or transposons. Many of the multi-antibiotic resistance plasmids that are frequently found in clinical collections have determinants of mercury resistance as well. Furthermore, mercury resistance is a consistent component of the chromosomal resistance determinant of MRS A (methicillin-resistant Staphylococcus aureus), a current clinical problem and one with no apparent connection to the use of mercurials. If the mercury resistance system is present, the expression of the detoxifying activities is tightly regulated and turned on only when needed. The MerR regulatory protein turns on mRNA synthesis by a positive activator mechanism (for primary references, see Silver and Walderhaug 1992 Misra 1992). 

Furukawa, N. and K. Tonomura. 1971. Enzyme system involved in the decomposition of phenyl mercuric acetate by mercury-resistant Pseudomonas. Agric. Biol. Chem. 35 604-610. 

Errzymatic conversion of the metal to a form which is less toxic for the bacterium, e.g. CHsHgand Hg (Misra 1992). Many of these metal-resistance mechanisms are encoded by genetic systems which have been extensively studied and are well understood. Perhaps the best-studied metal-resistance system is encoded by genes of the mer, or mercury resistance, operon. In this system, Hg(II) is transported into the cell via the MerT transporter protein, and detoxified by reduction to less toxic volatile mercury by an intracellular mercury reductase, MerA (see Osborn et al. 1997, Hobman etal. 2000). 

Furukawa, K., and K. Tonoraura, 1971, Eri2yme Systems Involved in the DeconpDsition of Phenyl Mercuric Acetate ly Mercury-Resistant PseudOTionas, Agricultural and Biological Chemistry 35, pp. 

We have reviewed the best-known prokaryotic plasmid-based resistance systems from the view of metal ion homeostasis. Plasmid-based systems are, with few exceptions, inducible and are regulated by activators (mercury and copper) or repressors (cadmium) in different cases. The metal-binding motifs used by these systems are frequently localized (e.g., Cys-Xaa-Xaa-Cys, Glu-His-His, and His-Xaa-His), but occasionally coordinating cysteine residues on different subunits interact (as in the MerR regulatory protein and mercuric reductase enzyme). 

Thus films can be divided into two groups according to their morphology. Discontinuous films are porous, have a low resistance and are formed at potentials close to the equilibrium potential of the corresponding electrode of the second kind. They often have substantial thickness (up to 1 mm). Films of this kind include halide films on copper, silver, lead and mercury, sulphate films on lead, iron and nickel oxide films on cadmium, zinc and magnesium, etc. Because of their low resistance and the reversible electrode reactions of their formation and dissolution, these films are often very important for electrode systems in storage batteries. 

Various systems have been used to measure the mercury level change in the stem during intrusion. Indirect methods include resistance or capacitance measurements along the stem of the dilatometer [39], These readings are taken in conjunction with pressure readings, to correlate the number of pores at a specific pore size. 

To circumvent some of the above-mentioned drawbacks of sulfur-based mercury chemodosimeters, a system based on the alkyne oxymercuration of 58 has been developed (Fig. 22) [146]. 58 shows high selectivity, a limit of detection of ca. 8 ppm, resistance against strong oxidants, and a positive reaction even in the presence of cysteine, which is known to form stable mercury complexes and is used for the extraction of mercury from tissue samples. Another metal that is well-known for its catalytic ability is palladium, catalyzing different reactions depending on its oxidation state. Since this metal is toxic, assessment of the maximum allowable concentration of Pd in consumer products such as pharmaceuticals requires highly sensitive and selective detection schemes. For this purpose, indicator 60 was conceived to undergo allylic oxidative insertion to the fluorescein... 

The calorimetric thermometer measures temperature changes within the calorimeter bucket. It must be able to provide excellent resolution and repeatability. High single-point accuracy is not required since it is the change in temperature that is important in fuel calorimetry. Mercurial thermometers, platinum resistance thermometers, quartz oscillators, and thermistor systems have all been successfully used as calorimelric thermometers. 

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