Transposon A-generated mutations in the mercuric resistance genes of plasmid R100-1

Abstract

A series of 23 transposon 801(Tn801)-induced mutations of plasmid R100-1 from mercuric salts resistance to sensitivity was studied. Although Tn801 transposed frequently into the mer region of the plasmid, fine structural analysis showed that the site of insertion within mer varied. About one-half of the Tn801 insertion events also caused a deletion of greater than 1 megadalton. Genetic and restriction endonuclease EcoRI and BamHI analysis of the mutant plasmid deoxyribonucleic acid elucidated the organization of the mer operon and suggested the existence of a transacting regulatory factor governing resistance to mercuric salts. Tn801 insertions leading to mercuric sensitivity occurred in the restriction endonuclease fragments EcoRI-H and EcoRI-I. Regulatory mutations leading to a 50-fold-reduced synthesis of mercuric reductase enzyme occurred in two complementation classes thought to represent the gene for a transacting inducer molecule and a cis-acting operator-promoter sequence. Mutations leading to total loss of the enzyme mercuric reductase occurred on both the EcoRI-H and EcoRI-I fragments, showing that the structural gene for this enzyme (merA) bridges the EcoRI cleavage site separating the segments. Hypersensitivity to mercuric salts resulted when Tn801 insertion occurred in the reductase gene in the operatordistal portion of the operon. Hypersensitive cells inducibly bound three to five

Molecular mechanisms of copper resistance and accumulation in bacteria

“…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].…”

Bacterial mercury resistance from atoms to ecosystems

“…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).…”

“…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).…”

Some mercurial resistance plasmids from different incompatibility groups specify merR regulatory functions that both repress and induce the mer operon of plasmid R100

“…Genetic analysis of the narrow-spectrum determinant of plasmid R100 revealed the following operon structure (8,14,16). A regulatory gene merR specifies a protein which controls transcription of the merTCA operon both positively and negatively.…”

“…However, several observations suggest that the basic genetic organization might be quite similar to that of R100 or TnSOJ, with additional Hg2 -inducible genes specifying the organomercurial lyase subunits. (i) Plasmid R828, which specifies broad-spectrum Hgr, complemented a merR mutant of R100, stimulating induction of the mer operon (8).…”