Functional coexistence of twin arsenic resistance systems in Pseudomonas putida KT2440. - Related Documents




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18701.0000Functional coexistence of twin arsenic resistance systems in Pseudomonas putida KT2440. The genome of the soil bacterium Pseudomonas putida KT2440 bears two virtually identical arsRBCH operons putatively encoding resistance to inorganic arsenic species. Single and double chromosomal deletions in each of these ars clusters of this bacterium were tested for arsenic sensitivity and found that the contribution of each operon to the resistance to the metalloid was not additive, as either cluster sufficed to endow cells with high-level resistance. However, otherwise identical traits linked to each of the ars sites diverged when temperature was decreased. Growth of the various mutants at 15°C (instead of the standard 30°C for P. putida) uncovered that ars2 affords a much higher resistance to As (III) than the ars1 counterpart. Reverse transcription polymerase chain reaction of arsB1 and arsB2 genes as well as lacZ fusions to the Pars1 and Pars2 promoters traced the difference to variations in transcription of the corresponding gene sets at each temperature. Functional redundancy may thus be selected as a stable condition - rather than just as transient state - if it affords one key activity to be expressed under a wider range of physicochemical settings. This seems to provide a straightforward solution to regulatory problems in environmental bacteria that thrive under changing scenarios.201524673935
932710.9996Detection of the merA gene and its expression in the environment. Bacterial transformation of mercury in the environment has received much attention owing to the toxicity of both the ionic form and organomercurial compounds. Bacterial resistance to mercury and the role of bacteria in mercury cycling have been widely studied. The genes specifying the required functions for resistance to mercury are organized on the mer operon. Gene probing methodologies have been used for several years to detect specific gene sequences in the environment that are homologous to cloned mer genes. While mer genes have been detected in a wide variety of environments, less is known about the expression of these genes under environmental conditions. We combined new methodologies for recovering specific gene mRNA transcripts and mercury detection with a previously described method for determining biological potential for mercury volatilization to examine the effect of mercury concentrations and nutrient availability on rates of mercury volatilization and merA transcription. Levels of merA-specific transcripts and Hg(II) volatilization were influenced more by microbial activity (as manipulated by nutrient additions) than by the concentration of total mercury. The detection of merA-specific transcripts in some samples that did not reduce Hg(II) suggests that rates of mercury volatilization in the environment may not always be proportional to merA transcription.19968849424
633920.9996Novel acid resistance genes from the metagenome of the Tinto River, an extremely acidic environment. Microorganisms that thrive in acidic environments are endowed with specialized molecular mechanisms to survive under this extremely harsh condition. In this work, we performed functional screening of six metagenomic libraries from planktonic and rhizosphere microbial communities of the Tinto River, an extremely acidic environment, to identify genes involved in acid resistance. This approach has revealed 15 different genes conferring acid resistance to Escherichia coli, most of which encoding putative proteins of unknown function or previously described proteins not known to be related to acid resistance. Moreover, we were able to assign function to one unknown and three hypothetical proteins. Among the recovered genes were the ClpXP protease, the transcriptional repressor LexA and nucleic acid-binding proteins such as an RNA-binding protein, HU and Dps. Furthermore, nine of the retrieved genes were cloned and expressed in Pseudomonas putida and Bacillus subtilis and, remarkably, most of them were able to expand the capability of these bacteria to survive under severe acid stress. From this set of genes, four presented a broad-host range as they enhance the acid resistance of the three different organisms tested. These results expand our knowledge about the different strategies used by microorganisms to survive under extremely acid conditions.201323145860
29230.9996Mechanisms underlying expression of Tn10 encoded tetracycline resistance. Tetracycline-resistance determinants encoding active efflux of the drug are widely distributed in gram-negative bacteria and unique with respect to genetic organization and regulation of expression. Each determinant consists of two genes called tetA and tetR, which are oriented with divergent polarity, and between them is a central regulatory region with overlapping promoters and operators. The amino acid sequences of the encoded proteins are 43-78% identical. The resistance protein TetA is a tetracycline/metal-proton antiporter located in the cytoplasmic membrane, while the regulatory protein TetR is a tetracycline inducible repressor. TetR binds via a helix-turn-helix motif to the two tet operators, resulting in repression of both genes. A detailed model of the repressor-operator complex has been proposed on the basis of biochemical and genetic data. The tet genes are differentially regulated so that repressor synthesis can occur before the resistance protein is expressed. This has been demonstrated for the Tn10-encoded tet genes and may be a common property of all tet determinants, as suggested by the similar locations of operators with respect to promoters. Induction is mediated by a tetracycline-metal complex and requires only nanomolar concentrations of the drug. This is the most sensitive effector-inducible system of transcriptional regulation known to date. The crystal structure of the TetR-tetracycline/metal complex shows the Tet repressor in the induced, non-DNA binding conformation. The structural interpretation of many noninducible TetR mutants has offered insight into the conformational changes associated with the switch between inducing and repressing structures of TetR. Tc is buried in the core of TetR, where it is held in place by multiple contacts to the protein.19947826010
18940.9995Arsenate detoxification in a Pseudomonad hypertolerant to arsenic. Pseudomonas sp. strain As-1, obtained from an electroplating industrial effluent, was capable of growing aerobically in growth medium supplemented with up to 65 mM arsenate (As (V)), significantly higher concentrations than those tolerated by other reference arsenic resistant bacteria. The majority of the arsenic was detected in culture supernatants as arsenite (As (III)) and X-ray absorbance spectroscopy suggested that 30% of this cell-bound arsenic was As (V), 65% As (III) and 5% of arsenic was associated with sulphur. PCR analysis using primers designed against arsenic resistance genes of other Gram-negative bacteria confirmed the presence of an arsenic resistance operon comprising of three genes, arsR, arsB and arsC in order of predicted transcription, and consistent with a role in intracellular reduction of As (V) and efflux of As (III). In addition to this classical arsenic resistance mechanism, other biochemical responses to arsenic were implicated. Novel arsenic-binding proteins were purified from cellular fractions, while proteomic analysis of arsenic-induced cultures identified the upregulation of additional proteins not normally associated with the metabolism of arsenic. Cross-talk with a network of proteins involved in phosphate metabolism was suggested by these studies, consistent with the similarity between the phosphate and arsenate anions.200717160678
900750.9995Genes involved in copper resistance influence survival of Pseudomonas aeruginosa on copper surfaces. AIMS: To evaluate the killing of Pseudomonas aeruginosa PAO1 on copper cast alloys and the influence of genes on survival on copper containing medium and surfaces. METHODS AND RESULTS: Different strains of P. aeruginosa were inoculated on copper containing medium or different copper cast alloys and the survival rate determined. The survival rates were compared with rates on copper-free medium and stainless steel as control. In addition, the effect of temperature on survival was examined. CONCLUSIONS: Copper cast alloys had been previously shown to be bactericidal to various bacteria, but the mechanism of copper-mediated killing is still not known. In this report, we demonstrate that P. aeruginosa PAO1 is rapidly killed on different copper cast alloys and that genes involved in conferring copper resistance in copper-containing medium also influenced survival on copper cast alloys. We also show that the rate of killing is influenced by temperature. SIGNIFICANCE AND IMPACT OF THE STUDY: To use copper surfaces more widely as bactericidal agents in various settings, it is important to understand how genes influence survival on these surfaces. Here we show that genes shown to be involved in copper resistance in P. aeruginosa PAO1 can have an impact on the length of survival time on copper cast alloys under certain conditions. This is an important first step for evaluation of future use of copper surfaces as bactericidal agents.200919239551
17160.9995Codon usage bias reveals genomic adaptations to environmental conditions in an acidophilic consortium. The analysis of codon usage bias has been widely used to characterize different communities of microorganisms. In this context, the aim of this work was to study the codon usage bias in a natural consortium of five acidophilic bacteria used for biomining. The codon usage bias of the consortium was contrasted with genes from an alternative collection of acidophilic reference strains and metagenome samples. Results indicate that acidophilic bacteria preferentially have low codon usage bias, consistent with both their capacity to live in a wide range of habitats and their slow growth rate, a characteristic probably acquired independently from their phylogenetic relationships. In addition, the analysis showed significant differences in the unique sets of genes from the autotrophic species of the consortium in relation to other acidophilic organisms, principally in genes which code for proteins involved in metal and oxidative stress resistance. The lower values of codon usage bias obtained in this unique set of genes suggest higher transcriptional adaptation to living in extreme conditions, which was probably acquired as a measure for resisting the elevated metal conditions present in the mine.201829742107
44570.9995Selection of Shigella flexneri candidate virulence genes specifically induced in bacteria resident in host cell cytoplasm. We describe an in vivo expression technology (IVET)-like approach, which uses antibiotic resistance for selection, to identify Shigella flexneri genes specifically activated in bacteria resident in host cell cytoplasm. This procedure required construction of a promoter-trap vector containing a synthetic operon between the promoterless chloramphenicol acetyl transferase (cat) and lacZ genes and construction of a library of plasmids carrying transcriptional fusions between S. flexneri genomic fragments and the cat-lacZ operon. Clones exhibiting low levels (<10 micro g ml-1) of chloramphenicol (Cm) resistance on laboratory media were analysed for their ability to induce a cytophatic effect--plaque--on a cell monolayer, in the presence of Cm. These clones were assumed to carry a plasmid in which the cloned fragment acted as a promoter/gene which is poorly expressed under laboratory conditions. Therefore, only strains harbouring fusion-plasmids in which the cloned promoter was specifically activated within host cytoplasm could survive within the cell monolayer in the presence of Cm and give a positive result in the plaque assay. Pai (plaque assay induced) clones, selected following this procedure, were analysed for intracellular (i) beta-galactosidase activity, (ii) proliferation in the presence of Cm, and (iii) Cm resistance. Sequence analysis of Pai plasmids revealed genes encoding proteins of three functional classes: external layer recycling, adaptation to microaerophilic environment and gene regulation. Sequences encoding unknown functions were also trapped and selected by this new IVET-based protocol.200212390353
845680.9995Identification of genes required by Bacillus thuringiensis for survival in soil by transposon-directed insertion site sequencing. Transposon-directed insertion site sequencing was used to identify genes required by Bacillus thuringiensis to survive in non-axenic plant/soil microcosms. A total of 516 genetic loci fulfilled the criteria as conferring survival characteristics. Of these, 127 (24.6 %) were associated with uptake and transport systems; 227 loci (44.0 %) coded for enzymatic properties; 49 (9.5 %) were gene regulation or sensory loci; 40 (7.8 %) were structural proteins found in the cell envelope or had enzymatic activities related to it and 24 (4.7 %) were involved in the production of antibiotics or resistance to them. Eighty-three (16.1 %) encoded hypothetical proteins or those of unknown function. The ability to form spores was a key survival characteristic in the microcosms: bacteria, inoculated in either spore or vegetative form, were able to multiply and colonise the soil, whereas a sporulation-deficient mutant was not. The presence of grass seedlings was critical to colonisation. Bacteria labelled with green fluorescent protein were observed to adhere to plant roots. The sporulation-specific promoter of spo0A, the key regulator of sporulation, was strongly activated in the rhizosphere. In contrast, the vegetative-specific promoters of spo0A and PlcR, a pleiotropic regulator of genes with diverse activities, were only very weakly activated.201424310935
18890.9995Resistance to ag(i) cations in bacteria: environments, genes and proteins. Bacterial resistance to Ag(I) has been reported periodically with isolates from many environments where toxic levels of silver might be expected to occur, but initial reports were limited to the occurrence of resistant bacteria. The availability of silver-resistance conferring DNA sequences now allow genetic and mechanistic studies that had basically been missing. The genes determining Ag(I) resistance were sequenced from a plasmid found in a burn ward isolate. The 14.2 kb determinant contains seven recognized genes, arranged in three mRNA transcriptional units. The silE gene determines an extracellular (periplasmic space) metal-binding protein of 123 amino acids, including ten histidine residues implicated in Ag(I) binding. SilE is homologous to PcoE, of copper resistance. The next two genes, silR and silS, determine a two protein, histidine-kinase membrane sensor and aspartyl phosphate transcriptional responder, similar to other two component systems such as CzcR and CzcS (for cadmium, zinc and cobalt resistance) and PcoR and PcoS (for copper resistance). The remaining four genes, silCBAP, are co-transcribed and appear to determine Ag(+) efflux, with SilCBA homologous to CzcCBA, a three component cation/proton antiporter, and SilP a novel P-type ATPase with a amino-terminal histidine-rich cation-specificity region. The effects of increasing Ag(+) concentrations and growth medium halides (Cl-, Br- and I-) have been characterized, with lower Cl- concentrations facilitating resistance and higher concentrations toxicity. The properties of this unique Ag(I)-binding SilE protein are being characterized. Sequences similar to the silver-resistance DNA are being characterized by Southern blot DNA/DNA hybridization, PCR in vitro DNA synthesis and DNA sequencing. More than 25 additional closely related sequences have been identified in bacteria from diverse sources. Initial DNA sequencing results shows approximately 5-20% differences in DNA sequences.199918475907
682100.9995Comparative transcriptome analysis of Brucella melitensis in an acidic environment: Identification of the two-component response regulator involved in the acid resistance and virulence of Brucella. Brucella melitensis, encounters a very stressful environment in phagosomes, especially low pH levels. So identifying the genes that contribute to the replication and survival within an acidic environment is critical in understanding the pathogenesis of the Brucella bacteria. In our research, comparative transcriptome with RNA-seq were used to analyze the changes of genes in normal-medium culture and in pH4.4-medium culture. The results reveal that 113 genes expressed with significant differences (|log2Ratio| ≥ 3); about 44% genes expressed as up-regulated. With GO term analysis, structural constituent of the ribosome, rRNA binding, structural molecule activity, and cation-transporting ATPase activity were significantly enriched (p-value ≤ 0.05). These genes distributed in 51 pathways, in which ribosome and photosynthesis pathways were significantly enriched. Six pathways (oxidative phosphorylation, iron-transporting, bacterial secretion system, transcriptional regulation, two-component system, and ABC transporters pathways) tightly related to the intracellular survival and virulence of Brucella were analyzed. A two-component response regulator gene in the transcriptional regulation pathway, identified through gene deletion and complementary technologies, played an important role in the resistance to the acid-resistance and virulence of Brucella.201626691825
9304110.9995Variation of the flagellin gene locus of Campylobacter jejuni by recombination and horizontal gene transfer. The capacity of Campylobacter jejuni to generate genetic diversity was determined for its flagellar region. Recombination within a genome, as well as recombination after the uptake of exogenous DNA, could be demonstrated. The subunit of the flagellar filament of C. jejuni is encoded by two tandem genes, flaA and flaB, which are highly similar and therefore subject to recombination. A spontaneous recombination within this locus was demonstrated in a bacterial clone containing an antibiotic-resistance gene inserted in flaA. A recombinant was isolated in which the antibiotic-resistance gene had been repositioned into flaB, indicating that genetic information can be exchanged between the two flagellin genes of C. jejuni. The occurrence of recombinational events after the uptake of exogenous DNA by naturally competent bacteria was demonstrated with two mutants containing different antibiotic-resistance markers in their flagellin genes. Double-resistant transformants were formed when purified chromosomal donor DNA was added to a recipient strain, when the two bacterial cultures were mixed under conditions that induce natural competence, or when the two strains were cocultured. Both mechanisms of recombination may be used by the pathogenic organism to escape the immunological responses of the host or otherwise adapt to the environment.19957894725
314120.9995Characterization 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.201121838857
8680130.9995Environmental pH affects transcriptional responses to cadmium toxicity in Escherichia coli K-12 (MG1655). It has been widely reported that pH mediates cadmium toxicity to bacteria. We used a tripartite approach to investigate mechanisms by which pH affects cadmium toxicity that included analyses of: (1) growth kinetics, (2) global gene expression, and (3) cadmium speciation. Cadmium extended the lag phase at pH 7, but not at pH 5. DNA microarray analysis revealed that stress response genes including hdeA, otsA, and yjbJ were more highly expressed at pH 5 than at pH 7 after only 5 min of exposure to cadmium, suggesting that acidic pH more rapidly induced genes that confer cadmium resistance. In addition, genes involved in transport and many hypothetical genes were more highly expressed at pH 5 than at pH 7 in the presence of cadmium. Concentrations of two cadmium species, including one previously implicated in the mechanism by which pH mediates cadmium toxicity (CdOH+), increased with pH. Our data demonstrate that transcriptional responses of Escherichia coli to cadmium are substantially affected by pH and suggest that several stress response, transport, and hypothetical genes play roles in the mechanism by which pH mediates cadmium toxicity.200919220470
6340140.9995Identification and functional analysis of novel protein-encoding sequences related to stress-resistance. Currently, industrial bioproducts are less competitive than chemically produced goods due to the shortcomings of conventional microbial hosts. Thus, is essential developing robust bacteria for improved cell tolerance to process-specific parameters. In this context, metagenomic approaches from extreme environments can provide useful biological parts to improve bacterial robustness. Here, in order to build genetic constructs that increase bacterial resistance to diverse stress conditions, we recovered novel protein-encoding sequences related to stress-resistance from metagenomic databases using an in silico approach based on Hidden-Markov-Model profiles. For this purpose, we used metagenomic shotgun sequencing data from microbial communities of extreme environments to identify genes encoding chaperones and other proteins that confer resistance to stress conditions. We identified and characterized 10 novel protein-encoding sequences related to the DNA-binding protein HU, the ATP-dependent protease ClpP, and the chaperone protein DnaJ. By expressing these genes in Escherichia coli under several stress conditions (including high temperature, acidity, oxidative and osmotic stress, and UV radiation), we identified five genes conferring resistance to at least two stress conditions when expressed in E. coli. Moreover, one of the identified HU coding-genes which was retrieved from an acidic soil metagenome increased E. coli tolerance to four different stress conditions, implying its suitability for the construction of a synthetic circuit directed to expand broad bacterial resistance.202337840709
688150.9995The cop operon is required for copper homeostasis and contributes to virulence in Streptococcus pneumoniae. High levels of copper are toxic and therefore bacteria must limit free intracellular levels to prevent cellular damage. In this study, we show that a number of pneumococcal genes are differentially regulated by copper, including an operon encoding a CopY regulator, a protein of unknown function (CupA) and a P1-type ATPase, CopA, which is conserved in all sequenced Streptococcus pneumoniae strains. Transcriptional analysis demonstrated that the cop operon is induced by copper in vitro, repressed by the addition of zinc and is autoregulated by the copper-responsive CopY repressor protein. We also demonstrate that the CopA ATPase is a major pneumococcal copper resistance mechanism and provide the first evidence that the CupA protein plays a role in copper resistance. Our results also show that copper homeostasis is important for pneumococcal virulence as the expression of the cop operon is induced in the lungs and nasopharynx of intranasally infected mice, and a copA(-) mutant strain, which had decreased growth in high levels of copper in vitro, showed reduced virulence in a mouse model of pneumococcal pneumonia. Furthermore, using the copA(-) mutant we observed for the first time in any bacteria that copper homeostasis also appears to be required for survival in the nasopharynx.201121736642
6217160.9995Identification of the sigmaB regulon of Bacillus cereus and conservation of sigmaB-regulated genes in low-GC-content gram-positive bacteria. The alternative sigma factor sigma(B) has an important role in the acquisition of stress resistance in many gram-positive bacteria, including the food-borne pathogen Bacillus cereus. Here, we describe the identification of the set of sigma(B)-regulated genes in B. cereus by DNA microarray analysis of the transcriptome upon a mild heat shock. Twenty-four genes could be identified as being sigma(B) dependent as witnessed by (i) significantly lower expression levels of these genes in mutants with a deletion of sigB and rsbY (which encode the alternative sigma factor sigma(B) and a crucial positive regulator of sigma(B) activity, respectively) than in the parental strain B. cereus ATCC 14579 and (ii) increased expression of these genes upon a heat shock. Newly identified sigma(B)-dependent genes in B. cereus include a histidine kinase and two genes that have predicted functions in spore germination. This study shows that the sigma(B) regulon of B. cereus is considerably smaller than that of other gram-positive bacteria. This appears to be in line with phylogenetic analyses where sigma(B) of the B. cereus group was placed close to the ancestral form of sigma(B) in gram-positive bacteria. The data described in this study and previous studies in which the complete sigma(B) regulon of the gram-positive bacteria Bacillus subtilis, Listeria monocytogenes, and Staphylococcus aureus were determined enabled a comparison of the sets of sigma(B)-regulated genes in the different gram-positive bacteria. This showed that only three genes (rsbV, rsbW, and sigB) are conserved in their sigma(B) dependency in all four bacteria, suggesting that the sigma(B) regulon of the different gram-positive bacteria has evolved to perform niche-specific functions.200717416654
262170.9995Genome scanning in Haemophilus influenzae for identification of essential genes. We have developed a method for identifying essential genes by using an in vitro transposition system, with a small (975 bp) insertional element containing an antibiotic resistance cassette, and mapping these inserts relative to the deduced open reading frames of Haemophilus influenzae by PCR and Southern analysis. Putative essential genes are identified by two methods: mutation exclusion or zero time analysis. Mutation exclusion consists of growing an insertional library and identifying open reading frames that do not contain insertional elements: in a growing population of bacteria, insertions in essential genes are excluded. Zero time analysis consists of monitoring the fate of individual insertions after transformation in a growing culture: the loss of inserts in essential genes is observed over time. Both methods of analysis permit the identification of genes required for bacterial survival. Details of the mutant library construction and the mapping strategy, examples of mutant exclusion, and zero time analysis are presented.199910438768
170180.9995Effect of arsenite and growth in biofilm conditions on the evolution of Thiomonas sp. CB2. Thiomonas bacteria are ubiquitous at acid mine drainage sites and play key roles in the remediation of water at these locations by oxidizing arsenite to arsenate, favouring the sorption of arsenic by iron oxides and their coprecipitation. Understanding the adaptive capacities of these bacteria is crucial to revealing how they persist and remain active in such extreme conditions. Interestingly, it was previously observed that after exposure to arsenite, when grown in a biofilm, some strains of Thiomonas bacteria develop variants that are more resistant to arsenic. Here, we identified the mechanisms involved in the emergence of such variants in biofilms. We found that the percentage of variants generated increased in the presence of high concentrations of arsenite (5.33 mM), especially in the detached cells after growth under biofilm-forming conditions. Analysis of gene expression in the parent strain CB2 revealed that genes involved in DNA repair were upregulated in the conditions where variants were observed. Finally, we assessed the phenotypes and genomes of the subsequent variants generated to evaluate the number of mutations compared to the parent strain. We determined that multiple point mutations accumulated after exposure to arsenite when cells were grown under biofilm conditions. Some of these mutations were found in what is referred to as ICE19, a genomic island (GI) carrying arsenic-resistance genes, also harbouring characteristics of an integrative and conjugative element (ICE). The mutations likely favoured the excision and duplication of this GI. This research aids in understanding how Thiomonas bacteria adapt to highly toxic environments, and, more generally, provides a window to bacterial genome evolution in extreme environments.202033034553
442190.9995Mercuric reductase in environmental gram-positive bacteria sensitive to mercury. According to existing data, mercury resistance operons (mer operons) are in general thought to be rare in bacteria, other than those from mercury-contaminated sites. We have found that a high proportion of strains in environmental isolates of Gram-positive bacteria express mercuric reductase (MerA protein): the majority of these strains are apparently sensitive to mercury. The expression of MerA was also inducible in all cases. These results imply the presence of phenotypically cryptic mer resistance operons, with both the merA (mercuric reductase) and merR (regulatory) genes still present, but the possible absence of the transport function required to complete the resistance mechanism. This indicates that mer operons or parts thereof are more widely spread in nature than is suggested by the frequency of mercury-resistant bacteria.19921427009