Abstract:The region of plasmid NR1 concerned with resistance to Hg2' and organomercurials consists of sequences found on restriction endonuclease fragments EcoRI-H and EcoRI-I. When both fragments were cloned together into a derivative of plasmid ColEl, the hybrid plasmid conferred properties indistinguishable from those of the parental plasmid, NR1: resistance to Hg2' and to the organomercurials merbromin and fluoresceinmercuric acetate and the inducible synthesis of the enzyme mercuric reductase. When fragment EcoRI-… Show more
“…This effect was not observed with either copC or copD expressed alone. Similar observations led to the discovery of a mercury transport system encoded by merT and merP associated with mercury resistance [30,31]. Preliminary uptake studies indicated that cells containing copCD accumulated more copper than cells without these genes or with either copC or copD alone [23].…”
An unusual mechanism of metal resistance is found in certain plant pathogenic strains of Pseudomonas syringae that are exposed to high levels of copper compounds used in disease control on agricultural crops. These bacteria accumulate blue Cu2+ ions in the periplasm and outer membrane. At least part of this copper sequestering activity is determined by copper-binding protein products of the copper resistance operon (cop). Potential copper-binding sites of the periplasmic CopA protein show conservation with type-1, type-2, and type-3 copper sites of several eukaryotic multi-copper oxidases. In addition to compartmentalization of copper in the periplasm, two components of the cop operon, copC and copD, appear to function in copper uptake into the cytoplasm. Copper resistance operons related to cop have been described in the related plant pathogen Xanthomonas campestris and in Escherichia coli, but these resistance systems may differ functionally from the Pseudomonas syringae system.
“…This effect was not observed with either copC or copD expressed alone. Similar observations led to the discovery of a mercury transport system encoded by merT and merP associated with mercury resistance [30,31]. Preliminary uptake studies indicated that cells containing copCD accumulated more copper than cells without these genes or with either copC or copD alone [23].…”
An unusual mechanism of metal resistance is found in certain plant pathogenic strains of Pseudomonas syringae that are exposed to high levels of copper compounds used in disease control on agricultural crops. These bacteria accumulate blue Cu2+ ions in the periplasm and outer membrane. At least part of this copper sequestering activity is determined by copper-binding protein products of the copper resistance operon (cop). Potential copper-binding sites of the periplasmic CopA protein show conservation with type-1, type-2, and type-3 copper sites of several eukaryotic multi-copper oxidases. In addition to compartmentalization of copper in the periplasm, two components of the cop operon, copC and copD, appear to function in copper uptake into the cytoplasm. Copper resistance operons related to cop have been described in the related plant pathogen Xanthomonas campestris and in Escherichia coli, but these resistance systems may differ functionally from the Pseudomonas syringae system.
“…Work on the mechanism of HgR led to identi¢cation of the key detoxi¢cation enzyme, MerA, the mercuric ion reductase [12], and also of a second enzyme, MerB, which split the carbon^Hg bond in such compounds as the disinfectant phenylmercuric acetate (PMA) and the fungicide methylmercury chloride, a potent neurotoxic agent [13]. Membrane and periplasmic proteins involved in the seemingly paradoxical inward transport of ionic mercury [14,15] were also identi¢ed [16,17]. The ¢rst sequences of HgR loci revealed proteins corresponding to these biochemical and physiological functions as well as a candidate regulatory gene (merR).…”
Section: A Brief History Of the Study Of Mercury Resistancementioning
confidence: 99%
“…The existence of an operon-speci¢c Hg(II) uptake system was ¢rst suggested on the basis of the Hg(II)-hypersensitive phenotype of merA mutants [14,15]; such variants were more sensitive to Hg(II) than cells lacking the operon altogether, consistent with there being some mechanism for bringing Hg(II) into the cell. In the absence of the mercuric reductase, the internalized Hg(II) is not de-toxi¢ed by reduction to Hg(q).…”
Section: Transportmentioning
confidence: 99%
“…This 116-residue (12.4-kDa) inner membrane protein is strongly predicted to have three transmembrane helices, the ¢rst of which has a cysteine pair which would lie within the ¢rst hydrophobic helix and perhaps be accessible from the periplasmic side [157,162,178]. Mutations altering either of these cysteines decrease the Hg(II)-hypersensitive phenotype that is a surrogate for physical measurement of Hg(II) uptake [15]. As noted above, a Cys14Ser mutation in MerP blocks uptake of Hg(II) by wild-type MerT [169], perhaps by formation of a stable Hg(II)-bound trigonal structure involving the MerP Cys17 and MerT's N-terminus-proximal cysteine pair.…”
Section: Mertmentioning
confidence: 99%
“…Thus, studies of Hg(II) uptake are only done in hypersensitive mutants lacking the reductase but capable of expressing the inducible transport genes. Not surprisingly, these strains are limited to taking up only as much 203 Hg(II) as can saturate their intracellular Hg(II) binding capacity without poisoning themselves [15,188]. However, initial rates of uptake by induced hypersensitive merA mutants are considerably faster than 203 Hg(II) uptake by cells lacking the mer operon and it is on such rates that work has been done to dissect the energetic basis of uptake.…”
Section: Energy Source For Mercury Transportmentioning
Bacterial resistance to inorganic and organic mercury compounds (HgR) is one of the most widely observed phenotypes in eubacteria. Loci conferring HgR in Gram-positive or Gram-negative bacteria typically have at minimum a mercuric reductase enzyme (MerA) that reduces reactive ionic Hg(II) to volatile, relatively inert, monoatomic Hg(0) vapor and a membrane-bound protein (MerT) for uptake of Hg(II) arranged in an operon under control of MerR, a novel metal-responsive regulator. Many HgR loci encode an additional enzyme, MerB, that degrades organomercurials by protonolysis, and one or more additional proteins apparently involved in transport. Genes conferring HgR occur on chromosomes, plasmids, and transposons and their operon arrangements can be quite diverse, frequently involving duplications of the above noted structural genes, several of which are modular themselves. How this very mobile and plastic suite of proteins protects host cells from this pervasive toxic metal, what roles it has in the biogeochemical cycling of Hg, and how it has been employed in ameliorating environmental contamination are the subjects of this review.
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