Biochemical characterization of HgCl2-inducible polypeptides encoded by the mer operon of plasmid R100

Structural and Biochemical Characterization of Organotin and Organolead Compounds Binding to the Organomercurial Lyase MerB Provide New Insights into Its Mechanism of Carbon-Metal Bond Cleavage

The organomercurial lyase MerB has the unique ability to cleave carbon-Hg bonds, and structural studies indicate that three residues in the active site (C96, D99, and C159 in E. coli MerB) play important roles in the carbon-Hg bond cleavage. However, the role of each residue in carbon-metal bond cleavage has not been well-defined. To do so, we have structurally and biophysically characterized the interaction of MerB with a series of organotin and organolead compounds. Studies with two known inhibitors of MerB, dimethyltin (DMT) and triethyltin (TET), reveal that they inhibit by different mechanisms. In both cases the initial binding is to D99, but DMT subsequently binds to C96, which induces a conformation change in the active site. In contrast, diethyltin (DET) is a substrate for MerB and the Sn^IV product remains bound in the active site in a coordination similar to that of Hg^II following cleavage of organomercurial compounds. The results with analogous organolead compounds are similar in that trimethyllead (TML) is not cleaved and binds only to D99, whereas diethyllead (DEL) is a substrate and the Pb^IV product remains bound in the active site. Binding and cleavage is an exothermic reaction, while binding to D99 has negligible net heat flow. These results show that initial binding of organometallic compounds to MerB occurs at D99 followed, in some cases, by cleavage and loss of the organic moieties and binding of the metal ion product to C96, D99, and C159. The N-terminus of MerA is able to extract the bound Pb^VI but not the bound Sn^IV. These results suggest that MerB could be utilized for bioremediation applications, but certain organolead and organotin compounds may present an obstacle by inhibiting the enzyme.

Structural and Biochemical Characterization of a Copper-Binding Mutant of the Organomercurial Lyase MerB: Insight into the Key Role of the Active Site Aspartic Acid in Hg-Carbon Bond Cleavage and Metal Binding Specificity

In bacterial resistance to mercury, the organomercurial lyase (MerB) plays a key role in the detoxification pathway through its ability to cleave Hg-carbon bonds. Two cysteines (C96 and C159; Escherichia coli MerB numbering) and an aspartic acid (D99) have been identified as the key catalytic residues, and these three residues are conserved in all but four known MerB variants, where the aspartic acid is replaced with a serine. To understand the role of the active site serine, we characterized the structure and metal binding properties of an E. coli MerB mutant with a serine substituted for D99 (MerB D99S) as well as one of the native MerB variants containing a serine residue in the active site (Bacillus megaterium MerB2). Surprisingly, the MerB D99S protein copurified with a bound metal that was determined to be Cu(II) from UV-vis absorption, inductively coupled plasma mass spectrometry, nuclear magnetic resonance, and electron paramagnetic resonance studies. X-ray structural studies revealed that the Cu(II) is bound to the active site cysteine residues of MerB D99S, but that it is displaced following the addition of either an organomercurial substrate or an ionic mercury product. In contrast, the B. megaterium MerB2 protein does not copurify with copper, but the structure of the B. megaterium MerB2-Hg complex is highly similar to the structure of the MerB D99S-Hg complexes. These results demonstrate that the active site aspartic acid is crucial for both the enzymatic activity and metal binding specificity of MerB proteins and suggest a possible functional relationship between MerB and its only known structural homologue, the copper-binding protein NosL.

Mercury resistance and mercuric reductase activities and expression among chemotrophic thermophilic Aquificae

Mercury (Hg) resistance (mer) by the reduction of mercuric to elemental Hg is broadly distributed among the Bacteria and Archaea and plays an important role in Hg detoxification and biogeochemical cycling. MerA is the protein subunit of the homodimeric mercuric reductase (MR) enzyme, the central function of the mer system. MerA sequences in the phylum Aquificae form the deepest-branching lineage in Bayesian phylogenetic reconstructions of all known MerA homologs. We therefore hypothesized that the merA homologs in two thermophilic Aquificae, Hydrogenobaculum sp. strain Y04AAS1 (AAS1) and Hydrogenivirga sp. strain 128-5-R1-1 (R1-1), specified Hg resistance. Results supported this hypothesis, because strains AAS1 and R1-1 (i) were resistant to >10 μM Hg(II), (ii) transformed Hg(II) to Hg(0) during cellular growth, and (iii) possessed Hg-dependent NAD(P)H oxidation activities in crude cell extracts that were optimal at temperatures corresponding with the strains' optimal growth temperatures, 55°C for AAS1 and 70°C for R1-1. While these characteristics all conformed with the mer system paradigm, expression of the Aquificae mer operons was not induced by exposure to Hg(II) as indicated by unity ratios of merA transcripts, normalized to gyrA transcripts for hydrogen-grown AAS1 cultures, and by similar MR specific activities in thiosulfate-grown cultures with and without Hg(II). The Hg(II)-independent expression of mer in the deepest-branching lineage of MerA from bacteria whose natural habitats are Hg-rich geothermal environments suggests that regulated expression of mer was a later innovation likely in environments where microorganisms were intermittently exposed to toxic concentrations of Hg.

Characterization of mercury bioremediation by transgenic bacteria expressing metallothionein and polyphosphate kinase

Background

The use of transgenic bacteria has been proposed as a suitable alternative for mercury remediation. Ideally, mercury would be sequestered by metal-scavenging agents inside transgenic bacteria for subsequent retrieval. So far, this approach has produced limited protection and accumulation. We report here the development of a transgenic system that effectively expresses metallothionein (mt-1) and polyphosphate kinase (ppk) genes in bacteria in order to provide high mercury resistance and accumulation.

Results

In this study, bacterial transformation with transcriptional and translational enhanced vectors designed for the expression of metallothionein and polyphosphate kinase provided high transgene transcript levels independent of the gene being expressed. Expression of polyphosphate kinase and metallothionein in transgenic bacteria provided high resistance to mercury, up to 80 μM and 120 μM, respectively. Here we show for the first time that metallothionein can be efficiently expressed in bacteria without being fused to a carrier protein to enhance mercury bioremediation. Cold vapor atomic absorption spectrometry analyzes revealed that the mt-1 transgenic bacteria accumulated up to 100.2 ± 17.6 μM of mercury from media containing 120 μM Hg. The extent of mercury remediation was such that the contaminated media remediated by the mt-1 transgenic bacteria supported the growth of untransformed bacteria. Cell aggregation, precipitation and color changes were visually observed in mt-1 and ppk transgenic bacteria when these cells were grown in high mercury concentrations.

Conclusion

The transgenic bacterial system described in this study presents a viable technology for mercury bioremediation from liquid matrices because it provides high mercury resistance and accumulation while inhibiting elemental mercury volatilization. This is the first report that shows that metallothionein expression provides mercury resistance and accumulation in recombinant bacteria. The high accumulation of mercury in the transgenic cells could present the possibility of retrieving the accumulated mercury for further industrial applications.

Phytoremediation of mercury and organomercurials in chloroplast transgenic plants: enhanced root uptake, translocation to shoots, and volatilization

Transgenic tobacco plants engineered with bacterial merA and merB genes via the chloroplast genome were investigated to study the uptake, translocation of different forms of mercury (Hg) from roots to shoots, and their volatilization. Untransformed plants, regardless of the form of Hg supplied, reached a saturation point at 200 microM of phenylmercuric acetate (PMA) or HgCl2, accumulating Hg concentrations up to 500 microg g(-1) with significant reduction in growth. In contrast, chloroplast transgenic lines continued to grow well with Hg concentrations in root tissues up to 2000 microg g(-1). Chloroplasttransgenic lines accumulated both the organic and inorganic Hg forms to levels surpassing the concentrations found in the soil. The organic-Hg form was absorbed and translocated more efficiently than the inorganic-Hg form in transgenic lines, whereas no such difference was observed in untransformed plants. Chloroplast-transgenic lines showed about 100-fold increase in the efficiency of Hg accumulation in shoots compared to untransformed plants. This is the first report of such high levels of Hg accumulation in green leaves or tissues. Transgenic plants attained a maximum rate of elemental-Hg volatilization in two days when supplied with PMA and in three days when supplied with inorganic-Hg, attaining complete volatilization within a week. The combined expression of merAB via the chloroplast genome enhanced conversion of Hg2+ into Hg,0 conferred tolerance by rapid volatilization and increased uptake of different forms of mercury, surpassing the concentrations found in the soil. These investigations provide novel insights for improvement of plant tolerance and detoxification of mercury.

Phytoremediation of organomercurial compounds via chloroplast genetic engineering

Mercury (Hg), especially in organic form, is a highly toxic pollutant affecting plants, animals, and man. In plants, the primary target of Hg damage is the chloroplast; Hg inhibits electron transport and photosynthesis. In the present study, chloroplast genetic engineering is used for the first time to our knowledge to enhance the capacity of plants for phytoremediation. This was achieved by integrating a native operon containing the merA and merB genes (without any codon modification), which code for mercuric ion reductase (merA) and organomercurial lyase (merB), respectively, into the chloroplast genome in a single transformation event. Stable integration of the merAB operon into the chloroplast genome resulted in high levels of tolerance to the organomercurial compound, phenylmercuric acetate (PMA) when grown in soil containing up to 400 micro M PMA; plant dry weights of the chloroplast transformed lines were significantly higher than those of wild type at 100, 200, and 400 micro M PMA. That the merAB operon was stably integrated into the chloroplast genome was confirmed by polymerase chain reaction and Southern-blot analyses. Northern-blot analyses revealed stable transcripts that were independent of the presence or absence of a 3'-untranslated region downstream of the coding sequence. The merAB dicistron was the more abundant transcript, but less abundant monocistrons were also observed, showing that specific processing occurs between transgenes. The use of chloroplast transformation to enhance Hg phytoremediation is particularly beneficial because it prevents the escape of transgenes via pollen to related weeds or crops and there is no need for codon optimization to improve transgene expression. Chloroplast transformation may also have application to other metals that affect chloroplast function.

Bacterial mercury resistance from atoms to ecosystems

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.

MerF is a mercury transport protein: different structures but a common mechanism for mercuric ion transporters?

Mercury resistance determinants are widespread in Gram-negative bacteria, but vary in the number and identity of genes present. We have shown that the merF gene from plasmid pMER327/419 encodes a 8.7 kDa mercury transport protein, by determining in vivo mercury volatilisation when MerF is expressed in the presence of mercuric reductase. We have confirmed that MerC of Tn21 is also a mercuric ion transporter. We have been able to detect interaction of the periplasmic protein MerP only with the MerT transporter, and not with MerF or MerC. Hydropathy analysis led to the prediction of models for MerT, MerC and MerF having three, four and two transmembrane regions respectively. In all three cases one pair of cysteine residues is predicted to be within the inner membrane with a second pair of cysteine residues on the cytoplasmic face, and the second helix contains a proline and at least one charged residue. The mechanisms of mercuric ion transport may be similar in these transporters even though their structures in the membrane differ.

A mercuric ion uptake role for the integral inner membrane protein, MerC, involved in bacterial mercuric ion resistance

Bacterial detoxification of mercuric ion depends on the presence of one or more integral membrane proteins (MerT and/or MerC) whose postulated function is in transport of Hg2+ from a periplasmic Hg2+-binding protein (MerP) to cytoplasmic mercuric reductase. In this study, MerC from the Tn21-encoded mer operon was overexpressed and studied in vesicles and in purified form to clarify the role played by this protein in mercuric ion resistance. MerC-containing vesicles were found to take up mercuric ion independently of MerP. Since uptake correlated with the level of MerC expression was unaffected by osmotic pressure, and was only partially decreased in the presence of 0.05% Triton X-100, the observed uptake appears to represent mainly binding to MerC. Binding was inhibited by thiol-specific reagents, consistent with an essential role for cysteine residues. The essential thiol groups were inaccessible to hydrophilic thiol reagents, whereas hydrophobic reagents completely abolished Hg2+ binding. These observations are consistent with the predicted topology of the protein, wherein all 4 cysteine residues are either in the cytoplasm or the bilayer. A role for MerC in Hg2+ transport is thus also likely. Based on these results, a modified model for bacterial Hg2+ transport is proposed.

Lack of involvement of merT and merP in methylmercury transport in mercury resistant Pseudomonas K-62

To clarify whether the merT and merP genes play a role in the transport of methylmercury, we constructed a deletion plasmid, pMRD141 which lacked the genes conferring the organomercurial lyase and the mercuric reductase from plasmid, pMRA17 containing the entire broad-spectrum mercury resistance determinants of the 26 kb-plasmid from Pseudomonas K-62. Plasmid, pMRD141 showed hypersensitivity to Hg2+ but still expressed a normal sensitivity to methylmercury. The mercury-induced hypersensitive cells carrying pMRD141 took up appreciably more 203Hg2+ than the induced resistant cells with pMRA17 and the sensitive cells with cloning vector, Bluescript II SK(+) but no difference in the uptake of CH3(203)Hg2+ among these three strains was found. These results suggested that the merT and merP are only involved in the Hg2+ transport but do not participate in the transport of methylmercury.

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.

Effect of gene amplification on mercuric ion reduction activity of Escherichia coli

The mercury resistance (mer) operon of plasmid R100 was cloned onto various plasmid vectors to study the effect of mer gene amplification on the rate of Hg2+ reduction by Escherichia coli cells. The plasmids were maintained at copy numbers ranging from 3 to 140 copies per cell. The overall Hg2+ reduction rate of intact cells increased only 2.4-fold for the 47-fold gene amplification. In contrast, the rate of the cytoplasmic reduction reaction, measured in permeabilized cells, increased linearly with increasing gene copy number, resulting in a 6.8-fold overall amplification. RNA hybridizations indicated that mRNA of the cytoplasmic mercuric reductase (merA gene product) increased 11-fold with the 47-fold gene amplification, while mRNA of the transport protein (merT gene product) increased only 5.4-fold. Radiolabeled proteins produced in maxicells were used to correlate the expression levels of the mer polypeptides with the measured reduction rates. The results indicated that, with increasing gene copy number, there was an approximately 5-fold increase in the merA gene product compared with a 2.5-fold increase in the merT gene product. These data demonstrate a parallel increase of Hg2+ reduction activity and transport protein expression in intact cells with plasmids with different copy numbers. In contrast, the expression level of the mercuric reductase gene underwent higher amplification than that of the transport genes at both the RNA and protein levels as plasmid copy number increased.

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.

Thiobacillus ferrooxidans mer operon: sequence analysis of the promoter and adjacent genes

The merA upstream nucleotide (nt) sequence (1378 bp) of the Hg2(+)-resistance-encoding gene system (mer) in Thiobacillus ferrooxidans, was determined. The region contains two open reading frames: unidentified reading frame 1 (URF1) and merC. URF1 has 63-73% homology with those of Tn501, R100 and pDU1358, although the corresponding product has not yet been identified. Thiobacillus merC had 61% and 55% homology with R100 merC at the nt and amino acid (aa) sequence levels, respectively, and its product, consisting of 143 aa, was highly hydrophobic. No sequence homologous to merR, merT. merP or merD of R100 were observed on either strand. Within the 70-100-bp sequence upstream from the merC start codon. there was a sequence highly homologous to the promoter of merT of other Gram- mer systems. From primer extension and Northern-blot analyses, it became clear that merC and merA were co-transcribed from this putative transcription start point. The mer transcript in T. ferrooxidans was only detected in Hg2(+)-induced cells. Therefore, it was concluded that the T. ferrooxidans mer system is an inducible operon.

Transport systems encoded by bacterial plasmids

A variety of bacterial functions are encoded on plasmids, extrachromosomal elements. Examples of plasmid-borne functions are antibiotic production and resistance, degradation of recalcitrant chemicals, virulence factors, and plant symbiotic properties. Several transport systems with diverse functions have recently been found to be carried on plasmids. These systems serve to either accumulate or extrude a compound from a cell. The focus of this review is to present a survey on several of these novel plasmid-borne transport systems emphasizing functions, components, and molecular genetics.

DNA sequence analysis of bacterial toxic heavy metal resistances

Bacterial plasmids have genes that confer highly specific resistances to As, Bi, Cd, Cu, Cr, Hg, Pb, Te, Zn, and other toxic heavy metals. For each toxic cation or anion, generally a different resistance system exists, and these systems may be "linked" together on multiple resistance plasmids. For Cd2+, AsO2-, AsO4(3)-, Hg2+, and organomercurials, DNA sequence analysis has supplemented direct physiological and biochemical experiments to produce sophisticated understanding. The cadA ATPase of S. aureus plasmids is a 727 amino acid membrane ATPase that pumps Cd2+ from the cells as rapidly as it is accumulated. This polypeptide is related by sequence to other cation translocating ATPases, including the membrane K+ ATPases of Escherichia coli and Streptococcus faecalis, the H+ ATPases of yeast and Neurospora, the Na+/K+ ATPases of vertebrate animals, and the Ca2+ ATPases of rabbit muscle. The conserved residues include the aspartyl residue that is phosphorylated, the lysine involved in ATP binding, and the proline within a membrane translocating region. The arsenate and arsenite translocating ATPase consists of 3 polypeptides (from DNA sequence analysis), including a recognizable ATP binding protein (arsA), an integral membrane protein (arsB gene), and a substrate specificity subunit (arsC gene). Inorganic mercury and organomercurial degradation is carried out by a series of about 6 polypeptides, including 2 soluble intracellular enzymes (organomercurial lyase and mercuric reductase). The latter is related by sequence and function to glutathione reductase and lipoamide dehydrogenase of prokaryotes and eukaryotes. These enzymes are dimeric, FAD-containing, NAD(P)H-dependent oxidoreductases. Other recognizable polypeptides in the mer system include a DNA-binding regulatory protein from the merR gene and a Hg2+ transport system consisting of a periplasmic Hg2(+)-binding protein (merP gene) and a membrane protein (merT gene) in gram negative systems.

Translation of merD in Tn21

All four sequenced examples of the mercury resistance (mer) operon of gram-negative bacteria have a promoter-distal reading frame, merD, whose removal has little effect on the resistance phenotype and whose translation has not previously been observed. Using merD-lacZ protein fusions, we show that merD is translated. However, Hg(II)-induced merD expression, as measured by beta-galactosidase activity and immunoblotting, is 10- to 15-fold lower than that of fusions to the gene immediately preceding it, merA.

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

The mercury resistance (mer) operons of the Gram-negative bacterial transposons, Tn21 and Tn501, are phenotypically indistinguishable and have extensive DNA identity. However, Tn21 mer has an additional coding region (merC) in the middle of the operon which is lacking in Tn501 and there is also a discrete region of the mercuric ion reductase gene (merA) which differs markedly between the two operons. DNA fragment probes were used to determine the distribution of specific mer coding regions in two distinct collections of mercury-resistant (Hgr) Gram-negative bacteria. Colony blot hybridization analysis showed that merC-positive operons occur almost exclusively in Escherichia, although merC-negative operons can also be found in this genus. The merC-negative operons were found in Citrobacter, Klebsiella, and Enterobacter and in some Pseudomonas. Most of the Pseudomonas did not hybridize detectably with either of the two operons studied, indicating that they harbor an unrelated or more distantly related class of mercury resistance locus. Southern hybridization patterns demonstrated that the merC-positive mer operon is well conserved at the DNA level, whereas the merC-negative operons are much less conserved. The presence of merC also correlated with conservation of a specific variant region of the merA gene and with an antibiotic resistance pattern similar to that of Tn21. Tn501 appears to be an atypical example of the merC-negative subgroup of Hgr loci.

Prediction of signal sequence-dependent protein translocation in bacteria: assessment of the Escherichia coli minicell system

The use of phenethyl alcohol (PEA) as a probe for signal sequence-dependent protein translocation in minicells was examined. Processing of beta-lactamases and tonA was inhibited by PEA at concentrations which did not affect production of the alpha and gamma forms of penicillin binding protein (PBP) lb. The PBPlbs are believed to lack leader sequences whereas the other proteins contain them. Processing of a beta-lactamase which shares the murein-lipoprotein export pathway was relatively resistant to PEA, consistent with previous findings in whole bacteria. The results reported here suggest that PEA is a suitable probe for leader sequences in the minicell system. By using PEA we predict that PBP4 does not require a leader sequence for membrane insertion.

Overexpression and DNA-binding properties of the mer-encoded regulatory protein from plasmid NR1 (Tn21)

In plasmid NR1 the expression of genes involved in mercury resistance (Tn21) is regulated by the trans-acting product of the merR gene. An in vivo T7 RNA polymerase-promoter overexpression system was used to detect a protein of approximately 16,000 daltons encoded by the merR reading frame. Overexpressed MerR constituted about 5% of labeled proteins. An in vitro MerR-mer-op (mer-op is the mer operator and promoter region) gel electrophoresis binding assay established that the binding site for MerR was located between the putative -35 and -10 sequences of the promoter for the mer structural genes. A nonsense mutation in the carboxyl half of MerR resulted in the loss of biological function and the loss of in vitro mer-op binding properties.

The genetics and biochemistry of mercury resistance

The ability of bacteria to detoxify mercurial compounds by reduction and volatilization is conferred by mer genes, which are usually plasmid located. The narrow spectrum (Hg2+ detoxifying) Tn501 and R100 determinants have been subjected to molecular genetic and DNA sequence analysis. Biochemical studies on the flavoprotein mercuric reductase have elucidated the mechanism of reduction of Hg2+ to Hg0. The mer genes have been mapped and sequenced and their protein products studied in minicells. Based on the deduced amino acid sequences, these proteins have been assigned a role in a mechanistic scheme for mercury flux in resistant bacteria. The mer genes are inducible, with regulatory control being exerted at the transcriptional level both positively and negatively. Attention is now focusing on broad-spectrum resistance involving detoxification of organomercurials by an additional enzyme, organomercurial lyase. Lyase genes have recently been cloned and sequencing studies are in progress.

Polypeptides specified by the mercuric resistance (mer) operon of plasmid R100

Overlapping deletion mutations were constructed in chimaeric plasmids carrying the mer operon of plasmid R100. Polypeptides specified by the mutant plasmids in Escherichia coli minicells correlated with the mer genes as follows: merT, 17- and 16-kDa polypeptides; merP, 9.8- and 9.5-kDa polypeptides; merC, a 14-kDa polypeptide; merA, 65- and 62-kDa polypeptides. The products of the merR and merD genes were not identified. The revised nomenclature of the mer genes is explained.

The nucleotide sequence of the mercuric resistance operons of plasmid R100 and transposon Tn501: further evidence for mer genes which enhance the activity of the mercuric ion detoxification system

The DNA sequences of the mercuric resistance determinants of plasmid R100 and transposon Tn501 distal to the gene (merA) coding for mercuric reductase have been determined. These 1.4 kilobase (kb) regions show 79% identity in their nucleotide sequence, and in both sequences two common potential coding sequences have been identified. In R100, the end of the homologous sequence is disrupted by an 11.2 kb segment of DNA which encodes the sulfonamide and streptomycin resistance determinants of Tn21. This insert contains terminal inverted repeat sequences and is flanked by a 5 base pair (bp) direct repeat. The first of the common potential coding sequences is likely to be that of the merD gene. Induction experiments and mercury volatilization studies demonstrate an enhancing but non-essential role for these merA-distal coding sequences in mercury resistance and volatilization. The potential coding sequences have predicted codon usages similar to those found in other Tn501 and R100 mer genes.

Physical characterization of the cloned protease III gene from Escherichia coli K-12

Analysis of the cloned protease III gene (ptr) from Escherichia coli K-12 has demonstrated that in addition to the previously characterized 110,000-Mr protease III protein, a second 50,000-Mr polypeptide (p50) is derived from the amino-terminal end of the coding sequence. The p50 polypeptide is found predominantly in the periplasmic space along with protease III, but does not proteolytically degrade insulin, a substrate for protease III. p50 does not appear to originate from autolysis of the larger protein. Protease III is not essential for normal cell growth since deletion of the structural gene causes no observed alterations in the phenotypic properties of the bacteria. A 30-fold overproduction of protease III does not affect cell viability. A simple new purification method for protease III is described.

Mercuric reductase structural genes from plasmid R100 and transposon Tn501: functional domains of the enzyme

The nucleotide sequence for the 2240 bp of plasmid R100 following the merC gene of the mercuric resistance operon has been determined and compared with the homologous sequence of transposon Tn501. The sequences following merC and preceding the next structural gene merA are unrelated between R100 and Tn501 and differ in length, with 72 bp in Tn501 and 509 bp in R100. The R100 sequence has a potential open reading frame (ORF) for a 140 amino acid polypeptide with a reasonable translational start signal preceding it. The merA genes contain 1686 (Tn501) and 1695 (R100) bp respectively. When optimally aligned, the merA sequences differ in 18% of their positions. These differences were clustered in specific regions. In addition, there was one nucleotide triplet in the Tn501 sequence which has no counterpart in the R100 sequence and one dodecyl-nucleotide sequence in the R100 sequence without counterpart in Tn501. Thus the predicted merA polypeptide of Tn501 contains 561 amino acids and the R100 counterpart contains 564 amino acids. Comparison of the R100 mercuric reductase sequences with that for human glutathione reductase [Krauth-Siegel et al.: Eur. J. Biochem. 121 (1982) 259-267], for which there is a 2 A resolution electron density map [Thieme et al.: J. Mol. Biol. 152 (1981) 763-782] shows a strong homology, with 26% identical amino acids and many conservative substitutions. This homology allows the conclusion that the active site of these enzymes and the contact positions for flavin adenine dinucleotide (FAD) and NADPH are highly conserved, while the amino- and carboxyl-terminal sequences differ.

Versatile mercury-resistant cloning and expression vectors

Cloning vectors have been constructed employing two diverse replicons, IncQ and P15A. Both vectors confer resistance to kanamycin (Km) and mercuric ions (Hg2+). One of these vectors, pDG105, is a broad-host-range, nonconjugative, oligocopy IncQ plasmid, which is capable of transforming Escherichia coli, Acinetobacter calcoaceticus, and Pseudomonas putida. The second vector, pDG106, is a narrow-host-range, multicopy cloning vector compatible with pBR322. Both vectors contain unique cloning sites in the Km-resistance gene for HindIII, SmaI, and XhoI, as well as unique EcoRI and ScaI sites in the mer operon. Cloning into the EcoRI site in the mer operon results in the mercury "supersensitive" phenotype, easily detectable by replica plating. Insertion of the galK gene into the EcoRI site in the mer operon results in Hg2+-inducible galactokinase activity, demonstrating the application of these plasmids as regulated expression vectors.

The structure of the mer operon

The DNA sequence has been determined for a 3.8-kb region which encodes the mercury-resistance (mer) operon of the IncFII plasmid NR1. The sequence reveals 4 open reading frames which could encode proteins of 12,522, 9,429, 14,965, and 58,912 d corresponding to the 4 previously described Hg-inducible proteins detected in minicells carrying mer+ plasmids. The Hg(II) reductase protein sequence is about 90% homologous to that of Tn501, but the DNA sequence shows a homology of 60-70% to that of Tn501 except for short regions of very high homology. The entire mer region is 63.4% G-C overall. The region encoding the merR (positive regulatory) function has 3 possible open reading frames, 2 of which overlap in one direction and the third of which reads in the opposite direction. Attempts to visualize the polypeptide(s) encoded by the merR cistron were unsuccessful.

Physical and genetic map of the organomercury resistance (Omr) and inorganic mercury resistance (Hgr) loci of the IncM plasmid R831b

Tn7 insertion mutagenesis has been used to facilitate the generation of a physical (restriction endonuclease) and genetic map of the IncM plasmid, R831b. The only selectable phenotypes carried by this 90-kb conjugative plasmid are resistances to inorganic mercury [Hg(II)] and to organomercury compounds. Mutants in the Hgr locus of R831b complemented previously described mutants in the mer operon of the IncFII plasmid R100, indicating functional homology of the locus in each of these different plasmids. However, the R831b Hgr locus is not notably similar in restriction site pattern to either the mer operon of R100 or the mercury resistance transposon, Tn501. Although the enzymes they encode are co-ordinately regulated, the Omr locus of R831b maps approx. 13.5 kb away from the Hgr locus. Three insertions which affect neither phenotype lie between the Hgr and Omr loci; thus, the loci are separated both physically and genetically. One mutant was obtained which tentatively identifies the position of the Tra locus of R831b as adjacent to the Hgr locus.

Mercuric ion-resistance operons of plasmid R100 and transposon Tn501: the beginning of the operon including the regulatory region and the first two structural genes

The mercuric ion-resistance operons of plasmid R100 (originally from Shigella) and transposon Tn501 (originally from a plasmid isolated in Pseudomonas) have been compared by DNA sequence analysis. The sequences for the first 1340 base pairs of Tn501 are given with the best alignment with the comparable 1319 base pairs of R100. The homology between the two sequences starts at base 58 after the end of the insertion sequence IS-1 of R100. The sequences include the transcriptional regulatory region, and the homology is particularly strong in regions just upstream from potential transcriptional initiation sites. The trans-acting regulatory gene merR consists of 180 base pairs in both cases and codes for a highly basic polypeptide of 60 amino acids, which is also rich in serine. The Tn501 and R100 merR genes differ in 25 of the 180 base positions, and the resulting polypeptides differ in seven amino acids. The regulatory region before the major transcription initiation site contains potential -35 and -10 sequences and dyad symmetrical sequences, which may be the merR binding sites for transcriptional regulation. The first structural gene, merT, encodes a highly hydrophobic polypeptide of 116 amino acids. The R100 and Tn501 merT genes differ in 17% of their positions, leading to 14 (12%) amino acid changes. This region had previously been shown to encode a protein governing membrane transport of mercuric ions. The second structural gene, merC, would give a 91 amino acid polypeptide with a hydrophobic amino-terminal segment. The Tn501 and R100 merC genes differ at 37 base positions, leading to 10 amino acid changes.

A second positive regulatory function in the mer (mercury resistance) operon

Transpositional mutagenesis of the mer operon of the IncFII plasmid, R100, has revealed a second, trans-acting positive regulatory function. Mutants in this function do not synthesize any of the three small mer operon peptides and have no inducible Hg(II) uptake activity. This second regulatory function is part of complementation group B and so depends upon the activity of the previously described trans-acting positive regulatory function merR. All mutants in this new function map in the amino-terminal 20 kDal of the Hg(II) reductase, suggesting either that this enzyme is also a regulatory protein or that there is a distinct protein whose reading frame is superimposed on that of the Hg(II) reductase. While we have only seen the five previously described mer operon peptides of 69, 66, 15.1, 14 and 12 (13) kDal encoded in minicells by single-copy plasmids, we have observed two new HgCl2-inducible polypeptides of approx. 20 kDal in minicells carrying a multicopy derivative of the mer operon of R100. Sequence data for the Hg(II) reductase region of the related mer operon of the transposon, Tn501 [Brown, N.L., Ford, S.J., Pridmore, R.D. and Fritzinger, D.C., Biochemistry 22 (1983) 4089-4095], shows a second reading frame very rich in cysteine and arginine which overlaps the amino-terminal 20 kDal of the Hg(II) reductase structural gene. We believe that this reading frame is the structural gene for this new regulatory function and propose the name merC (for control).

Tn5 insertion mutations in the mercuric ion resistance genes derived from plasmid R100

The mercuric resistance (mer) genes of plasmid R100 were cloned into plasmid pBR322. A series of transposon Tn5 insertion mutations in the mer genes were isolated and mapped. The mutants were characterized phenotypically by their sensitivity to Hg2+ and by binding and volatilization of 203Hg2+. Dominance and complementation tests were also performed. Mutations affecting the previously described mer genes merR (regulation), merT (transport), and merA (reductase) were characterized. Evidence was obtained for two new mer genes, which have been called merC and merD. A restriction enzyme map of the mer region was drawn with the gene order merRTCAD. Transcriptional merR-lac and merA-lac fusions were generated by insertion of phage Mu d amp lac into plasmid R100-1. These were used to study regulation of mer gene expression. The merR gene product appears to regulate negatively its own expression as well as acting as both a negative and a positive regulator of the merTCA genes.