The distribution and divergence of DNA sequences related to the Tn21 and Tn501 mer operons

The Mercury-Tolerant Microbiota of the Zooplankton Daphnia Aids in Host Survival and Maintains Fecundity under Mercury Stress

Many aquatic organisms can thrive in polluted environments by having the genetic capability to withstand suboptimal conditions. However, the contributions of microbiomes under these stressful environments are poorly understood. We investigated whether a mercury-tolerant microbiota can extend its phenotype to its host by ameliorating host survival and fecundity under mercury-stress. We isolated microbiota members from various clones of Daphnia magna, screened for the mercury-biotransforming merA gene, and determined their mercury tolerance levels. We then introduced the mercury-tolerant microbiota, Pseudomonas-10, to axenic D. magna and quantified its merA gene expression, mercury reduction capability, and measured its impact on host survival and fecundity. The expression of the merA gene was up-regulated in Pseudomonas-10, both in isolation and in host-association with mercury exposure. Pseudomonas-10 is also capable of significantly reducing mercury concentration in the medium. Notably, mercury-exposed daphnids containing only Pseudomonas-10 exhibited higher survival and fecundity than mercury-exposed daphnids supplemented with parental microbiome. Our study showed that zooplankton, such as Daphnia, naturally harbor microbiome members that are eco-responsive and tolerant to mercury exposure and can aid in host survival and maintain host fecundity in a mercury-contaminated environment. This study further demonstrates that under stressful environmental conditions, the fitness of the host can depend on the genotype and the phenotype of its microbiome.

Cloning, Expression, Isotope Labeling, and Purification of Transmembrane Protein MerF from Mercury Resistant Enterobacter sp. AZ-15 for NMR Studies

Mercury resistant (HgR) Enterobacter sp. AZ-15 was isolated from heavy metal polluted industrial wastewater samples near to districts Kasur and Sheikhupura, Pakistan. 16S rDNA ribotyping and phylogentic analysis showed 98% homology with already reported Enterobacter species. The merF gene encoding transmembrane protein-MerF was amplified from genomic DNA and ligated into pET31b+ vector using restriction endonucleases, SphI and XhoI. The genetic codons of merF gene encoding cysteine residues were mutated into codons, translating into serine residues by site-directed mutagenesis. Ketosteroid isomerase (KSI), a fusion tag which is present in pET31b+ vector, was used in the expression of merFm gene. KSI was used to drive the target peptide (MerFm) into inclusion bodies so that the peptide yield and purity were increased. The stable plasmid pET31b+:merFm was transformed into C43(DE3) E.coli cells. The high expression of uniformly 15N isotopically labeled-MerFm protein was induced with 1 mM IPTG. The purification of 15N-MerFm recombinant protein by Ni-NTA and size exclusion chromatography involved an unfolding/refolding procedure. The two-dimensional HSQC NMR spectra of MerFm protein showed the purity and correct number of resonances for each amide. 1H-15N HSQC NMR experiment also confirmed that no modification of the tryptophan residue occurred during cyanogen bromide cleavage. A small scale reservoir of Luria Bertani (LB) medium supplemented with 20 μg/ml of HgCl2 showed 90% detoxification of Hg by Enterobacter sp. AZ-15. The accumulation of Hg on the cell surface of this strain was visualized by scanning electron microscopy (SEM) which confirmed its potential use in Hg-bioremediation.

Mercurial-resistance determinants in Pseudomonas strain K-62 plasmid pMR68

We report the complete nucleotide sequence of plasmid pMR68, isolated from Pseudomonas strain K-62, two plasmids contribute to broad-spectrum mercury resistance and that the mer operon from one of them (pMR26) has been previously characterized. The plasmid was 71,020 bp in length and contained 75 coding regions. Three mer gene clusters were identified. The first comprised merR-orf4-orf5-merT1-merP1-merF-merA-merB1, which confers bacterial resistance to mercuric ions and organomercury. The second and third clusters comprised merT2-merP2, which encodes a mercury transport system, and merB2, which encodes an organomercurial lyase, respectively. The deduced amino acid sequences for the proteins encoded by each of the mer genes identified in pMR68 bore greater similarity to sequences from Methylobacterium extorquens AM1 than to those from pMR26, a second mercury-resistance plasmid from Pseudomonas strain K-62. Escherichia coli cells carrying pMKY12 (containing merR-orf4-orf5-merT1-merP1-merF-merA-merB1 cloned from pMR68) and cells carrying pMRA114 (containing merR-merT-merP-merA-merG-merB1 cloned from plasmid pMR26) were more resistant to, and volatilized more, mercury from mercuric ions and phenylmercury than the control cells. The present results, together with our earlier findings, indicate that the high phenylmercury resistance noted for Pseudomonas strain K-62 seems to be achieved by multiple genes, particularly by the multiple merB encoding organomercurial lyase and one merG encoding cellular permeability to phenylmercury. The novel mer gene identified in pMR68 may help us to design new strategies aimed at the bioremediation of mercurials.

Dissemination and persistence of blaCTX-M-9 are linked to class 1 integrons containing CR1 associated with defective transposon derivatives from Tn402 located in early antibiotic resistance plasmids of IncHI2, IncP1-alpha, and IncFI groups

This study analyzes the diversity of In60, a class 1 integron bearing CR1 and containing bla(CTX-M-9), and its association with Tn402, Tn21, and classical conjugative plasmids among 45 CTX-M-9-producing clinical strains (41 Escherichia coli strains, 2 Klebsiella pneumoniae strains, 1 Salmonella enterica strain, and 1 Enterobacter cloacae strain). Forty-five patients in a Spanish tertiary care hospital were studied (1996 to 2003). The diversity of In60 and association of In60 with Tn402 or mercury resistance transposons were investigated by overlapping PCR assays and/or hybridization. Plasmid characterization included comparison of restriction fragment length polymorphism patterns and determination of incompatibility group by PCR-based replicon typing, sequencing, and hybridization. CTX-M-9 plasmids belonged to IncHI2 (n = 26), IncP-1alpha (n = 10), IncFI (n = 4), and IncI (n = 1) groups. Genetic platforms containing bla(CTX-M-9) were classified in six types in relation to the In60 backbone and in eight subtypes in relation to Tn402 derivatives. They were associated with Tn21 sequences when located in IncP-1alpha or IncHI2 plasmids. Our study identified bla(CTX-M-9) in a high diversity of CR1-bearing class 1 integrons linked to different Tn402 derivatives, often to Tn21, highlighting the role of recombination events in the evolution of antibiotic resistance plasmids. The presence of bla(CTX-M-9) on broad-host-range IncP-1alpha plasmids might contribute to its dissemination to hosts that were not members of the family Enterobacteriaceae.

Mercury resistance transposons of gram-negative environmental bacteria and their classification

A total of 29 mercury resistance transposons were isolated from mercury-resistant environmental strains of proteobacteria collected in different parts of Eurasia and the USA and tested for hybridization with probes specific for transposase genes of known mercury resistance transposons. 9 were related to Tn21 in this test, 12 were related to Tn5053, 4 to Tn5041 and 1 to Tn5044; three transposons were negative in this test. Restriction mapping and DNA sequencing revealed that 12 transposons were identical or nearly identical to their corresponding relatives while the rest showed varying divergence from their closest relatives. Most of these previously unknown transposons apparently arose as a result of homologous or site-specific recombination. One of these, Tn5046, was completely sequenced, and shown to be a chimera with the mer operon and the transposition module derived from the transposons related to Tn5041 and to Tn5044, respectively. Transposon Tn5070, showing no hybridization with the specific probes used in this study, was also completely sequenced. The transposition module of Tn5070 was most closely related to that of Tn3 while the mer operon was most closely related to that of plasmid pMERPH. The merR of Tn5070 is transcribed in the same direction as the mer structural genes, which is typical for mer operons of gram-positive bacteria. Our data suggest that environmental bacteria may harbor many not yet recognized mercury resistance transposons and warrant their further inventory.

Characterization of mercury resistance mechanisms in marine sediment microbial communities

While estuarine sediments are often severely polluted with mercury, few studies have focused on the mechanisms of adaptation to mercury contamination in marine sediment microbial communities. In this study, we report a high frequency of Gram-negative bacterial isolates that are resistant to the heavy metal mercury obtained from the aerobic culturable marine microbial community. We detected a low frequency of genes homologous to mer(Tn21) in isolates from three out of four different estuarine environments. Other mercury resistant culturable bacterial isolates lacking homology to the known mer genes were able to reduce Hg(II) to its volatile Hg(0) form, indicating the presence of divergent mer genes. In addition, a number of mercury resistant isolates, obtained from three of the four marine sites investigated, exhibited decreased resistance to mercury in the presence of the protonophore carbonyl cyanide m-chlorophenylhydrazone. Representative mercury resistant bacterial isolates were identified by phylogenetic analysis as belonging to the alpha and gamma subclasses of the class Proteobacteria.

Re-evaluation of antibiotic and mercury resistance in Escherichia coli populations isolated in 1978 from Amazonian rubber tree tappers and Indians

A study was carried out to assess the stability of antimicrobial susceptibility of wild isolates upon long-term storage using fifty-three Escherichia coli strains isolated in 1978 from feces of healthy children from the Amazon region in Brazil, exposed to low levels of antimicrobial agents, and examined for resistance to mercury and four antibiotics. All of the strains were kept in Lignières medium at room temperature and were transferred to fresh media four times during this period. Thirty-five out of the 53 strains analyzed in 1978 were viable. Upon recovery, antibiotic and mercury resistance was estimated. All of the 35 strains maintained their original phenotype in a stable fashion, except for one multiresistant strain which became susceptible to kanamycin. Fifty-four percent of the strains exhibited a resistance phenotype, among which 47% had conjugative plasmids.

Horizontal spread of mer operons among gram-positive bacteria in natural environments

Horizontal dissemination of the genes responsible for resistance to toxic pollutants may play a key role in the adaptation of bacterial populations to environmental contaminants. However, the frequency and extent of gene dissemination in natural environments is not known. A natural horizontal spread of two distinct mercury resistance (mer) operon variants, which occurred amongst diverse Bacillus and related species over wide geographical areas, is reported. One mer variant encodes a mercuric reductase with a single N-terminal domain, whilst the other encodes a reductase with a duplicated N-terminal domain. The strains containing the former mer operon types are sensitive to organomercurials, and are most common in the terrestrial mercury-resistant Bacillus populations studied in this work. The strains containing the latter operon types are resistant to organomercurials, and dominate in a Minamata Bay mercury-resistant Bacillus population, previously described in the literature. At least three distinct transposons (related to a class II vancomycin-resistance transposon, Tn1546, from a clinical Enterococcus strain) and conjugative plasmids are implicated as mediators of the spread of these mer operons.

Association of mercury resistance with antibiotic resistance in the gram-negative fecal bacteria of primates

Gram-negative fecal bacterial from three longitudinal Hg exposure experiments and from two independent survey collections were examined for their carriage of the mercury resistance (mer) locus. The occurrence of antibiotic resistance was also assessed in both mercury-resistant (Hgr) and mercury-susceptible (Hgs) isolates from the same collections. The longitudinal studies involved exposure of the intestinal flora to Hg released from amalgam "silver" dental restorations in six monkeys. Hgr strains were recovered before the installation of amalgams, and frequently these became the dominant strains while amalgams were installed. Such persistent Hgr strains always carried the same mer locus throughout the experiments. In both the longitudinal and survey collections, certain mer loci were preferentially associated with one genus, whereas other mer loci were recovered from many genera. In general, strains with any mer locus were more likely to be multiresistant than were strains without mer loci; this clustering tendency was also seen for antibiotic resistance genes. However, the association of antibiotic multiresistance with mer loci was not random; regardless of source, certain mer loci occurred in highly multiresistant strains (with as many as seven antibiotic resistances), whereas other mer loci were found in strains without any antibiotic resistance. The majority of highly multiresistant Hgr strains also carried genes characteristic of an integron, a novel genetic element which enables the formation of tandem arrays of antibiotic resistance genes. Hgr strains lacking antibiotic resistance showed no evidence of integron components.

Distribution, diversity and evolution of the bacterial mercury resistance (mer) operon

Mercury and its compounds are distributed widely across the earth. Many of the chemical forms of mercury are toxic to all living organisms. However, bacteria have evolved mechanisms of resistance to several of these different chemical forms, and play a major role in the global cycling of mercury in the natural environment. Five mechanisms of resistance to mercury compounds have been identified, of which resistance to inorganic mercury (HgR) is the best understood, both in terms of the mechanisms of resistance to mercury and of resistance to heavy metals in general. Resistance to inorganic mercury is encoded by the genes of the mer operon, and can be located on transposons, plasmids and the bacterial chromosome. Such systems have a worldwide geographical distribution, and furthermore, are found across a wide range of both Gram-negative and Gram-positive bacteria from both natural and clinical environments. The presence of mer genes in bacteria from sediment cores suggest that mer is an ancient system. Analysis of DNA sequences from mer operons and genes has revealed genetic variation both in operon structure and between individual genes from different mer operons, whilst analysis of bacteria which are sensitive to inorganic mercury has identified a number of vestigial non-functional operons. It is hypothesised that mer, due to its ubiquity with respect to geographical location, environment and species range, is an ancient system, and that ancient bacteria carried genes conferring resistance to mercury in response to increased levels of mercury in natural environments, perhaps resulting from volcanic activity. Models for the evolution of both a basic mer operon and for the Tn21-related family of mer operons and transposons are suggested. The study of evolution in bacteria has recently become dominated by the generation of phylogenies based on 16S rRNA genes. However, it is important not to underestimate the roles of horizontal gene transfer and recombinational events in evolution. In this respect mer is a suitable system for evaluating phylogenetic methods which incorporate the effects of horizontal gene transfer. In addition, the mer operon provides a model system in the study of environmental microbiology which is useful both as an example of a genotype which is responsive to environmental pressures and as a generic tool for the development of new methodology for the analysis of bacterial communities in natural environments.

Phylogeny of mercury resistance (mer) operons of gram-negative bacteria isolated from the fecal flora of primates

Nine polymorphic mer loci carried by 185 gram-negative fecal bacterial strains from humans and nonhuman primates are described. The loci were characterized with specific intragenic and intergenic PCR primers to amplify distinct regions covering approximately 80% of the typical gram-negative mer locus. These loci were grouped phylogenetically with respect to each other and with respect to seven previously sequenced mer operons from gram-negative bacteria (the latter designated loci 1, 2, 3, 6, 7, 8, and delta 8 by us here for the purpose of this analysis). Six of the mer loci recovered from primates are similar either to these previously sequenced mer loci or to another locus recently observed in environmental isolates (locus 4), and three are novel (loci 5, 9, and 10). We have observed merC, or a merC-like gene, or merF on the 5' side of merA in all of the loci except that of Tn501 (here designated mer locus 6). The merB gene was observed occasionally, always on the 3' side of merA. Unlike the initial example of a merB-containing mer locus carried by plasmid pDU1358 (locus 8), all the natural primate loci carrying merB also had large deletions of the central region of the operon (and were therefore designated locus delta 8). Four of the loci we describe (loci 2, 5, 9, and 10) have no region of homology to merB from pDU1358 and yet strains carrying them were phenylmercury resistant. Two of these loci (loci 5 and 10) also lacked merD, the putative secondary regulator of operon expression. Phylogenetic comparison of character states derived from PCR product data grouped those loci which have merC into one clade; these are locus 1 (including Tn21), locus 3, and locus 4. The mer loci which lack merC grouped into a second clade: locus 6 (including Tn501) and locus 2. Outlying groups lacked merD or possessed merB. While these mer operons are characterized by considerable polymorphism, our ability to discern coherent clades suggests that recombination is not entirely random and indeed may be focused on the immediate 5' and 3' proximal regions of merA. Our observations confirm and extend the idea that the mer operon is a genetic mosaic and has a predominance of insertions and/or deletions of functional genes immediately before and after the merA gene. chi sites are found in several of the sequenced operons and may be involved in the abundant reassortments we observe for mer genes.

Bacterial heavy metal resistance: new surprises

Bacterial plasmids encode resistance systems for toxic metal ions including Ag+, AsO2-, AsO4(3-), Cd2+, CO2+, CrO4(2-), Cu2+, Hg2+, Ni2+, Pb2+, Sb3+, TeO3(2-), Tl+, and Zn2+. In addition to understanding of the molecular genetics and environmental roles of these resistances, studies during the last few years have provided surprises and new biochemical mechanisms. Chromosomal determinants of toxic metal resistances are known, and the distinction between plasmid resistances and those from chromosomal genes has blurred, because for some metals (notably mercury and arsenic), the plasmid and chromosomal determinants are basically the same. Other systems, such as copper transport ATPases and metallothionein cation-binding proteins, are only known from chromosomal genes. The largest group of metal resistance systems function by energy-dependent efflux of toxic ions. Some of the efflux systems are ATPases and others are chemiosmotic cation/proton antiporters. The CadA cadmium resistance ATPase of gram-positive bacteria and the CopB copper efflux system of Enterococcus hirae are homologous to P-type ATPases of animals and plants. The CadA ATPase protein has been labeled with 32P from gamma-32P-ATP and drives ATP-dependent Cd2+ uptake by inside-out membrane vesicles. Recently isolated genes defective in the human hereditary diseases of copper metabolism, Menkes syndrome and Wilson's disease, encode P-type ATPases that are more similar to the bacterial CadA and CopB ATPases than to eukaryote ATPases that pump different cations. The arsenic resistance efflux system transports arsenite, using alternatively either a two-component (ArsA and ArsB) ATPase or a single polypeptide (ArsB) functioning as a chemiosmotic transporter. The third gene in the arsenic resistance system, arsC, encodes an enzyme that converts intracellular arsenate [As (V)] to arsenite [As (III)], the substrate of the efflux system. The three-component Czc (Cd2+, Zn2+, and CO2+) chemiosmotic efflux pump of soil microbes consists of inner membrane (CzcA), outer membrane (CzcC), and membrane-spanning (CzcB) proteins that together transport cations from the cytoplasm across the periplasmic space to the outside of the cell. Finally, the first bacterial metallothionein (which by definition is a small protein that binds metal cations by means of numerous cysteine thiolates) has been characterized in cyanobacteria.

merA gene expression in aquatic environments measured by mRNA production and Hg(II) volatilization

The relationship of merA gene expression (specifying the enzyme mercuric reductase) to mercury volatilization in aquatic microbial communities was investigated with samples collected at a mercury-contaminated freshwater pond, Reality Lake, in Oak Ridge, Tenn. Levels of merA mRNA transcripts and the rate of inorganic mercury [Hg(II)] volatilization were related to the concentration of mercury in the water and to heterotrophic activity in field samples and laboratory incubations of pond water in which microbial heterotrophic activity and Hg(II) concentration were manipulated. Levels of merA-specific mRNA and Hg(II) volatilization were influenced more by microbial metabolic activity than by the concentration of mercury. merA-specific transcripts were detected in some samples which did not reduce Hg(II), suggesting that rates of mercury volatilization in environmental samples may not always be proportional to merA expression.

Cloning and expression of the mercury resistance genes of marine Pseudomonas sp. strain MR1 plasmid pMR1 in Escherichia coli

The mercury resistance determinant of marine Pseudomonas sp. strain MR1 plasmid pMR1 was cloned into a narrow-host-range vector pUC18. A direct selection of mercury resistant clones was successful and 12 clones were evolved; 9 from direct selection on mercury agar plates and 3 from ampicillin-resistant white colonies. All the predicted clones efficiently volatilized mercury. One of the hybrid plasmids pMRD5, containing a 15.5-kb insert, conferred inducible resistance to both HgCl2 and phenyl mercury acetate with over a 40-fold increase in mer resistance in Escherichia coli HB101. No DNA homology existed between the mer operon of Pseudomonas sp. strain MR1 and the characterized determinants of Tn501 mer DNA.

Polymerase chain reaction-restriction fragment length polymorphism analysis shows divergence among mer determinants from gram-negative soil bacteria indistinguishable by DNA-DNA hybridization

Mercury resistant (Hgr) bacteria were isolated from four terrestrial sites: three containing high levels of mercury (sites T2, SE, and SO) and one uncontaminated site (SB). The frequencies of Hgr bacteria in the total cultivable populations were 0.05% (SB), 0.69% (SO), 4.8% (SE), and 25% (T2). Between 35 and 100% of the isolates from the four sites contained DNA sequences homologous to a DNA probe from the mercury resistance (mer) operon of the Tn501 Hgr determinant. The mer sequences of 10 Tn501-homologous Hgr determinants from each site were amplified by the polymerase chain reaction, with primers designed to consensus sequences of the mer determinants of Tn501, Tn21, and pMJ100, and were classified on the basis of the size of the amplified product and the restriction fragment length polymorphism pattern. Two main groups of amplification product were identified. The first, represented by the T2 and SB isolates and one SE isolate, gave an amplification product indistinguishable in size from that amplified from Tn501 (approximately 1,010 bp). The second group, represented by the SO isolates and the majority of the SE isolates, produced larger amplification products of 1,040 or 1,060 bp. Restriction fragment length polymorphism analysis revealed that each amplification product size group could be further subdivided into five subgroups.

Mercury released from dental "silver" fillings provokes an increase in mercury- and antibiotic-resistant bacteria in oral and intestinal floras of primates

In a survey of 640 human subjects, a subgroup of 356 persons without recent exposure to antibiotics demonstrated that those with a high prevalence of Hg resistance in their intestinal floras were significantly more likely to also have resistance to two or more antibiotics. This observation led us to consider the possibility that mercury released from amalgam ("silver") dental restorations might be a selective agent for both mercury- and antibiotic-resistant bacteria in the oral and intestinal floras of primates. Resistances to mercury and to several antibiotics were examined in the oral and intestinal floras of six adult monkeys prior to the installation of amalgam fillings, during the time they were in place, and after replacement of the amalgam fillings with glass ionomer fillings (in four of the monkeys). The monkeys were fed an antibiotic-free diet, and fecal mercury concentrations were monitored. There was a statistically significant increase in the incidence of mercury-resistant bacteria during the 5 weeks following installation of the amalgam fillings and during the 5 weeks immediately following their replacement with glass ionomer fillings. These peaks in incidence of mercury-resistant bacteria correlated with peaks of Hg elimination (as high as 1 mM in the feces) immediately following amalgam placement and immediately after replacement of the amalgam fillings. Representative mercury-resistant isolates of three selected bacterial families (oral streptococci, members of the family Enterobacteriaceae, and enterococci) were also resistant to one or more antibiotics, including ampicillin, tetracycline, streptomycin, kanamycin, and chloramphenicol. While such mercury- and antibiotic-resistant isolates among the staphylococci, the enterococci, and members of the family Enterobacteriaceae have been described, this is the first report of mercury resistance in the oral streptococci. Many of the enterobacterial strains were able to transfer mercury and antibiotic resistances together to laboratory bacterial recipients, suggesting that the loci for these resistances are genetically linked. Our findings indicate that mercury released from amalgam fillings can cause an enrichment of mercury resistance plasmids in the normal bacterial floras of primates. Many of these plasmids also carry antibiotic resistance, implicating the exposure to mercury from dental amalgams in an increased incidence of multiple antibiotic resistance plasmids in the normal floras of nonmedicated subjects.

Roles of the Tn21 merT, merP, and merC gene products in mercury resistance and mercury binding

The mercury resistance (mer) operon of the gram-negative transposon Tn21 encodes not only a mercuric reductase and regulatory genes but also two inner membrane proteins (MerT and MerC) and a periplasmic protein (MerP). Although the merT, merP, and merC genes have been implicated in Hg(II) transport, the individual roles of these genes have not been established. We created in vitro precise deletion and frameshift mutations that eliminated each of the genes singly and in combination. Our results show that both merT and merP are required for Hg(II) binding but that merC is not. Both merT and merP are required for full expression of Hg(II) resistance, but loss of merP is less deleterious than loss of merT. Furthermore, mutations eliminating both merT and merP decrease resistance more than the single mutations do. In contrast, mutating merC had no effect on Hg(II) resistance. Both the merT and merP mutations increase the threshold Hg(II) concentration for induction of merA-lacZ transcriptional fusions and cause an increase in the maximal expression level. In contrast, the merC mutation had little effect on the threshold inducing concentration of Hg(II) but decreased the level of expression. Our results show that merT and merP alone are sufficient to specify a mercury transport system. The role of merC remains obscure.

Untwist and shout: a heavy metal-responsive transcriptional regulator

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Hybridization of transposon Tn501 for detection of mercury resistance sequences in a marine environment

Total genomic DNA isolated by concentrating seawater and mercury-resistant bacteria were hybridized with a mer probe to detect the presence of homologous DNA sequences in marine coastal waters of the Bay of Bengal, India. Coastal water extracts induced with mercury hybridized with the mer operon of transposon Tn501. Most of the mercury-resistant bacteria that volatilized mercury also contained homologous DNA sequences to the mer probe.

Bacterial resistances to inorganic mercury salts and organomercurials

Environmental and clinical isolates of mercury-resistant (resistant to inorganic mercury salts and organomercurials) bacteria have genes for the enzymes mercuric ion reductase and organomercurial lyase. These genes are often plasmid-encoded, although chromosomally encoded resistance determinants have been occasionally identified. Organomercurial lyase cleaves the C-Hg bond and releases Hg(II) in addition to the appropriate organic compound. Mercuric reductase reduces Hg(II) to Hg(O), which is nontoxic and volatilizes from the medium. Mercuric reductase is a FAD-containing oxidoreductase and requires NAD(P)H and thiol for in vitro activity. The crystal structure of mercuric ion reductase has been partially solved. The primary sequence and the three-dimensional structure of the mercuric reductase are significantly homologous to those of other flavin-containing oxidoreductases, e.g., glutathione reductase and lipoamide dehydrogenase. The active site sequences are the most conserved region among these flavin-containing enzymes. Genes encoding other functions have been identified on all mercury ion resistance determinants studied thus far. All mercury resistance genes are clustered into an operon. Hg(II) is transported into the cell by the products of one to three genes encoded on the resistance determinants. The expression of the operon is regulated and is inducible by Hg(II). In some systems, the operon is inducible by both Hg(II) and some organomercurials. In gram-negative bacteria, two regulatory genes (merR and merD) were identified. The (merR) regulatory gene is transcribed divergently from the other genes in gram-negative bacteria. The product of merR represses operon expression in the absence of the inducers and activates transcription in the presence of the inducers. The product of merD coregulates (modulates) the expression of the operon. Both merR and merD gene products bind to the same operator DNA. The primary sequence of the promoter for the polycistronic mer operon is not ideal for efficient transcription by the RNA polymerase. The -10 and -35 sequences are separated by 19 (gram-negative systems) or 20 (gram-positive systems) nucleotides, 2 or 3 nucleotides longer than the 17-nucleotide optimum distance for binding and efficient transcription by the Escherichia coli sigma 70-containing RNA polymerase. The binding site of MerR is not altered by the presence of Hg(II) (inducer). Experimental data suggest that the MerR-Hg(II) complex alters the local structure of the promoter region, facilitating initiation of transcription of the mer operon by the RNA polymerase. In gram-positive bacteria MerR also positively regulates expression of the mer operon in the presence of Hg(II).

Cloning and expression in Escherichia coli of mercuric ion resistance coding genes from Zymomonas mobilis

From a genomic library of Zymomonas mobilis prepared in Escherichia coli, two clones (carrying pZH4 and pZH5) resistant to the mercuric ion were isolated. On partial restriction analysis these two clones appeared to have the same 2.9 kb insert. Mercuric reductase activity was assayed from the Escherichia coli clone carrying pZH5 and it was Hg(2+)-inducible, NADH dependent and also required 2-mercaptoethanol for its activity. The plasmid pZH5 encoded three polypeptides, mercuric reductase (merA; 65 kDa), a transport protein (merT 18-17 kDa) and merC (15 kDa) as analysed by SDS-PAGE. Southern blot analysis showed the positive signal for the total DNA prepared from Hgr Z. mobilis but not with the Hgs strain which was cured for a plasmid (30 kb). These results were also confirmed by isolating this plasmid from Hgr Z. mobilis and transforming into E. coli. Moreover the plasmid pZH5 also hybridized with the mer probes derived from Tn21.

Distribution of DNA Sequences Encoding Narrow- and Broad-Spectrum Mercury Resistance

The distribution of DNA sequences homologous with three mer genes was determined in unselected and mercury-resistant water and sediment isolates. The maximum proportions of unselected bacterial isolates containing DNA hybridizing with the 358merA, 358merB, and 501merR probes, derived from gram-negative organisms, were 93.8, 21, and 100%, respectively. Up to 53.3% of mercury chloride-resistant isolates and 54% of methylmercury hydroxide-resistant isolates did not contain DNA homologous with 358merA or 358merB, respectively. Hybridizations performed at high and low stringencies demonstrated that divergence of the merA gene accounted for many of the mercury-resistant but probe-negative isolates. Sixteen mercury-resistant Bacillus spp. isolated from the least contaminated site all contained DNA homologous with 258merA, originally from a gram-positive organism, but only four hybridized weakly with 358merA. The results demonstrate the wide distribution of mercury resistance genes but, because of the diversity of genetic determinants, highlight the importance of using multiple detection techniques and gene probes derived from a variety of origins for such studies.

Constitutive synthesis of a transport function encoded by the Thiobacillus ferrooxidans merC gene cloned in Escherichia coli

Mercuric reductase activity determined by the Thiobacillus ferrooxidans merA gene (cloned and expressed constitutively in Escherichia coli) was measured by volatilization of 203Hg2+. (The absence of a merR regulatory gene in the cloned Thiobacillus mer determinant provides a basis for the constitutive synthesis of this system.) In the absence of the Thiobacillus merC transport gene, the mercury volatilization activity was cryptic and was not seen with whole cells but only with sonication-disrupted cells. The Thiobacillus merC transport function was compared with transport via the merT-merP system of plasmid pDU1358. Both systems, cloned and expressed in E. coli, governed enhanced uptake of 203Hg2+ in a temperature- and concentration-dependent fashion. Uptake via MerT-MerP was greater and conferred greater hypersensitivity to Hg2+ than did uptake with MerC. Mercury uptake was inhibited by N-ethylmaleimide but not by EDTA. Ag+ salts inhibited mercury uptake by the MerT-MerP system but did not inhibit uptake via MerC. Radioactive mercury accumulated by the MerT-MerP and by the MerC systems was exchangeable with nonradioactive Hg2+.

Hybridization of DNA probes with whole-community genome for detection of genes that encode microbial responses to pollutants: mer genes and Hg2+ resistance

Nucleic acids extracted from microbial biomass without prior culturing were hybridized with probes representing four mer operons to detect genes encoding adaptation to Hg2+ in whole-community genomes. A 29-fold enrichment in sequences similar to the mer genes of transposon Tn501 occurred during adaptation in a freshwater community. In an estuarine community, all four mer genes were only slightly enriched (by three- to fivefold), suggesting that additional, yet uncharacterized, mer genes encoded adaptation to Hg2+.