Nucleotide sequence and expression of the mercurial-resistance operon from Staphylococcus aureus plasmid pI258

Investigation of Staphylococcus aureus positive for Staphylococcal enterotoxin S and T genes

Staphylococcus aureus produces staphylococcal enterotoxins (SEs) and causes food poisoning. It is known that almost all SE-encoding genes are present on various types of mobile genetic elements and can mobilize among S. aureus populations. Further, plasmids comprise one of SE gene carriers. Previously, we reported novel SEs, SES and SET, harbored by the plasmid pF5 from Fukuoka5. In the present study, we analyzed the distribution of these SEs in various S. aureus isolates in Japan. We used 526 S. aureus strains and found 311 strains positive for at least one SE/SE-like toxin gene, but only two strains (Fukuoka5 and Hiroshima3) were positive for ses and set among the specimens. We analyzed two plasmids (pF5 and pH3) from these strains and found that they were different. Whereas these plasmids partially shared similar sequences involved in the ser/selj/set/ses gene cluster, other sequences were different. A comparison of these plasmids with those deposited in the NCBI database revealed that only one plasmid had the ser/selj/set/ses cluster with a stop mutation in set similar to that in pH3. In addition, the chromosomes of Fukuoka5 and Hiroshima3, positive for ses and set, were classified into different genotypes. Despite the low rate of gene positivity for these SEs, it is suggested that there is diversity in plasmids and strains carrying these two SEs. Consequently, regarding the entire feature of SE prevalence, we improved the multiplex PCR detection method for the SE superfamily to obtain further insight.

The Genome of Staphylococcus epidermidis O47

The skin colonizing coagulase-negative Staphylococcus epidermidis causes nosocomial infections and is an important opportunistic and highly adaptable pathogen. To gain more insight into this species, we sequenced the genome of the biofilm positive, methicillin susceptible S. epidermidis O47 strain (hereafter O47). This strain belongs to the most frequently isolated sequence type 2. In comparison to the RP62A strain, O47 can be transformed, which makes it a preferred strain for molecular studies. S. epidermidis O47’s genome has a single chromosome of about 2.5 million base pairs and no plasmid. Its oriC sequence has the same directionality as S. epidermidis RP62A, S. carnosus, S. haemolyticus, S. saprophyticus and is inverted in comparison to Staphylococcus aureus and S. epidermidis ATCC 12228. A phylogenetic analysis based on all S. epidermidis genomes currently available at GenBank revealed that O47 is closest related to DAR1907. The genome of O47 contains genes for the typical global regulatory systems known in staphylococci. In addition, it contains most of the genes encoding for the typical virulence factors for S. epidermidis but not for S. aureus with the exception of a putative hemolysin III. O47 has the typical S. epidermidis genetic islands and several mobile genetic elements, which include staphylococcal cassette chromosome (SCC) of about 54 kb length and two prophages φO47A and φO47B. However, its genome has no transposons and the smallest number of insertion sequence (IS) elements compared to the other known S. epidermidis genomes. By sequencing and analyzing the genome of O47, we provide the basis for its utilization in genetic and molecular studies of biofilm formation.

Microbial Diversity of Mer Operon Genes and Their Potential Rules in Mercury Bioremediation and Resistance

Background:

Mercury is a toxic metal that is present in small amounts in the environment, but its level is rising steadily, due to different human activities, such as industrialization. It can reach humans through the food chain, amalgam fillings, and other sources, causing different neurological disorders, memory loss, vision impairment, and may even lead to death; making its detoxification an urgent task.

Methods:

Various physical and chemical mercury remediation techniques are available, which generally aim at: (i) reducing its mobility or solubility; (ii) causing its vaporization or condensation; (iii) its separation from contaminated soils. Biological remediation techniques, commonly known as bioremediation, are also another possible alternative, which is considered as cheaper than the conventional means and can be accomplished using either (i) organisms harboring themeroperon genes (merB,merA,merR,merP,merT,merD,merF,merC,merE,merHandmerG), or (ii) plants expressing metal-binding proteins. Recently, differentmerdeterminants have been genetically engineered into several organisms, including bacteria and plants, to aid in detoxification of both ionic and organic forms of mercury.

Results:

Bacteria that are resistant to mercury compounds have at least a mercuric reductase enzyme (MerA) that reduces Hg+2to volatile Hg0, a membrane-bound protein (MerT) for Hg+2uptake and an additional enzyme, MerB, that degrades organomercurials by protonolysis. Presence of bothmerA andmerB genes confer broad-spectrum mercury resistance. However,merA alone confers narrow spectrum inorganic mercury resistance.

Conclusion:

To conclude, this review discusses the importance of mercury-resistance genes in mercury bioremediation. Functional analysis ofmeroperon genes and the recent advances in genetic engineering techniques could provide the most environmental friendly, safe, effective and fantastic solution to overcome mercuric toxicity.

Global analysis of transcriptional regulators in Staphylococcus aureus

Background

Staphylococcus aureus is a widely distributed human pathogen capable of infecting almost every ecological niche of the host. As a result, it is responsible for causing many different diseases. S. aureus has a vast array of virulence determinants whose expression is modulated by an intricate regulatory network, where transcriptional factors (TFs) are the primary elements. In this work, using diverse sequence analysis, we evaluated the repertoire of TFs and sigma factors in the community-associated methicillin resistant S. aureus (CA-MRSA) strain USA300-FPR3757.

Results

A total of 135 TFs and sigma factors were identified and classified into 36 regulatory families. From these around 43% have been experimentally characterized to date, which demonstrates the significant work still at hand to unravel the regulatory network in place for this important pathogen. A comparison of the TF repertoire of S. aureus against 1209 sequenced bacterial genomes was carried out allowing us to identify a core set of orthologous TFs for the Staphylococacceae, and also allowing us to assign potential functions to previously uncharacterized TFs. Finally, the USA300 TFs were compared to those in eleven other S. aureus strains including: Newman, COL, JH1, JH9, MW2, Mu3, Mu50, N315, RF122, MRSA252 and MSSA476. We identify conserved TFs among these strains and suggest possible regulatory interactions.

Conclusions

The analysis presented herein highlights the complexity of regulatory networks in S. aureus strains, identifies key conserved TFs among the Staphylococacceae, and offers unique insights into several as yet uncharacterized TFs.

The Genome of Nitrospina gracilis Illuminates the Metabolism and Evolution of the Major Marine Nitrite Oxidizer

In marine systems, nitrate is the major reservoir of inorganic fixed nitrogen. The only known biological nitrate-forming reaction is nitrite oxidation, but despite its importance, our knowledge of the organisms catalyzing this key process in the marine N-cycle is very limited. The most frequently encountered marine NOB are related to Nitrospina gracilis, an aerobic chemolithoautotrophic bacterium isolated from ocean surface waters. To date, limited physiological and genomic data for this organism were available and its phylogenetic affiliation was uncertain. In this study, the draft genome sequence of N. gracilis strain 3/211 was obtained. Unexpectedly for an aerobic organism, N. gracilis lacks classical reactive oxygen defense mechanisms and uses the reductive tricarboxylic acid cycle for carbon fixation. These features indicate microaerophilic ancestry and are consistent with the presence of Nitrospina in marine oxygen minimum zones. Fixed carbon is stored intracellularly as glycogen, but genes for utilizing external organic carbon sources were not identified. N. gracilis also contains a full gene set for oxidative phosphorylation with oxygen as terminal electron acceptor and for reverse electron transport from nitrite to NADH. A novel variation of complex I may catalyze the required reverse electron flow to low-potential ferredoxin. Interestingly, comparative genomics indicated a strong evolutionary link between Nitrospina, the nitrite-oxidizing genus Nitrospira, and anaerobic ammonium oxidizers, apparently including the horizontal transfer of a periplasmically oriented nitrite oxidoreductase and other key genes for nitrite oxidation at an early evolutionary stage. Further, detailed phylogenetic analyses using concatenated marker genes provided evidence that Nitrospina forms a novel bacterial phylum, for which we propose the name Nitrospinae.

Mobile genetic elements of Staphylococcus aureus

Bacteria such as Staphylococcus aureus are successful as commensal organisms or pathogens in part because they adapt rapidly to selective pressures imparted by the human host. Mobile genetic elements (MGEs) play a central role in this adaptation process and are a means to transfer genetic information (DNA) among and within bacterial species. Importantly, MGEs encode putative virulence factors and molecules that confer resistance to antibiotics, including the gene that confers resistance to beta-lactam antibiotics in methicillin-resistant S. aureus (MRSA). Inasmuch as MRSA infections are a significant problem worldwide and continue to emerge in epidemic waves, there has been significant effort to improve diagnostic assays and to develop new antimicrobial agents for treatment of disease. Our understanding of S. aureus MGEs and the molecules they encode has played an important role toward these ends and has provided detailed insight into the evolution of antimicrobial resistance mechanisms and virulence.

Organomercurials removal by heterogeneous merB genes harboring bacterial strains

Organomercury lyase (MerB) is a key enzyme in bacterial detoxification and bioremediation of organomercurials. However, the merB gene is often considered as an ancillary component of the mer operon because there is zero to three merB genes in different mer operons identified so far. In this study, organomercurials' removal abilities of native mercury-resistant bacteria that have one or multiple merB genes were examined. Each heterogeneous merB genes from these bacteria was further cloned into Escherichia coli to investigate the substrate specificity of each MerB enzyme. The merB1 gene from Bacillus megaterium MB1 conferred the highest volatilization ability to methylmercury chloride, ethylmercury chloride, thimerosal and p-chloromercuribenzoate, while the merB3 from B. megaterium MB1 conferred the fastest mercury volatilization activity to p-chloromercuribenzoate. The substrate specificities among these MerB enzymes show the necessity for selecting the appropriate bacteria strains or MerB enzymes to apply them in bioremediation engineering for cleaning up specific organomercurial contaminations.

A novel transposon, Tn6009, composed of a Tn916 element linked with a Staphylococcus aureus mer operon

OBJECTIVES

The aim of this study was to characterize a novel conjugative transposon Tn6009 composed of a Tn916 linked to a Staphylococcus aureus mer operon in representative Gram-positive and Gram-negative bacteria isolated in Nigeria and Portugal.

METHODS

Eighty-three Gram-positive and 34 Gram-negative bacteria were screened for the presence of the Tn6009 using DNA-DNA hybridization, PCR, hybridization of PCR products, sequencing and mating experiments by established procedures.

RESULTS

Forty-three oral and 23 urine Gram-negative and Gram-positive isolates carried the Tn6009. Sequencing was performed to verify the direct linkage between the mer resistance genes and the tet(M) gene. A Nigerian Klebsiella pneumoniae, isolated from a urinary tract infection patient, and one commensal isolate from each of the other Tn6009-positive genera, Serratia liquefaciens, Pseudomonas sp., Enterococcus sp. and Streptococcus sp. isolated from the oral and urine samples of healthy Portuguese children, were able to act as donors and conjugally transfer the Tn6009 to the Enterococcus faecalis JH2-2 recipient, resulting in tetracycline- and mercury-resistant E. faecalis transconjugants.

CONCLUSIONS

This study reports a novel non-composite conjugative transposon Tn6009 containing a Tn916 element linked to an S. aureus mer operon carrying genes coding for inorganic mercury resistance (merA), an organic mercury resistance (merB), a regulatory protein (merR) and a mercury transporter (merT). This transposon was identified in 66 isolates from two Gram-positive and three Gram-negative genera and is the first transposon in the Tn916 family to carry the Gram-positive mer genes directly linked to the tet(M) gene.

High diversity of bacterial mercuric reductase genes from surface and sub-surface floodplain soil (Oak Ridge, USA)

DNA was extracted from different depth soils (0-5, 45-55 and 90-100 cm below surface) sampled at Lower East Fork Poplar Creek floodplain (LEFPCF), Oak Ridge (TN, USA). The presence of merA genes, encoding the mercuric reductase, the key enzyme in detoxification of mercury in bacteria, was examined by PCR targeting Actinobacteria, Firmicutes or beta/gamma-Proteobacteria. beta/gamma-Proteobacteria merA genes were successfully amplified from all soils, whereas Actinobacteria were amplified only from surface soil. merA clone libraries were constructed and sequenced. beta/gamma-Proteobacteria sequences revealed high diversity in all soils, but limited vertical similarity. Less than 20% of the operational taxonomic units (OTU) (DNA sequences > or = 95% identical) were shared between the different soils. Only one of the 62 OTU was > or = 95% identical to a GenBank sequence, highlighting that cultivated bacteria are not representative of what is found in nature. Fewer merA sequences were obtained from the Actinobacteria, but these were also diverse, and all were different from GenBank sequences. A single clone was most closely related to merA of alpha-Proteobacteria. An alignment of putative merA genes of genome sequenced mainly marine alpha-Proteobacteria was used for design of merA primers. PCR amplification of soil alpha-Proteobacteria isolates and sequencing revealed that they were very different from the genome-sequenced bacteria (only 62%-66% identical at the amino-acid level), although internally similar. In light of the high functional diversity of mercury resistance genes and the limited vertical distribution of shared OTU, we discuss the role of horizontal gene transfer as a mechanism of bacterial adaptation to mercury.

The isolation and initial characterization of mercury resistant chemolithotrophic thermophilic bacteria from mercury rich geothermal springs

Mercury rich geothermal springs are likely environments where mercury resistance is critical to microbial life and where microbe-mercury interactions may have evolved. Eleven facultative thermophilic and chemolithoautotrophic, thiosulfate oxidizing bacteria were isolated from thiosulfate enrichments of biofilms from mercury rich hot sulfidic springs in Mount Amiata, Italy. Some strains were highly resistant to mercury (>or=200 muM HgCl(2)) regardless of its presence or absence during primary enrichments, and three reduced ionic mercury to its elemental form. The gene encoding for the mercuric reductase enzyme (MerA), was amplified by PCR from seven strains. However, one highly resistant strain did not reduce mercury nor carried merA, suggesting an alternative resistance mechanism. All strains were members of the order Bacillales and were most closely related to previously described thermophiles belonging to the Firmicutes. Phylogenetic analyses clustered the MerA of the isolates in two supported novel nodes within the Firmicutes lineage and a comparison with the 16S rRNA gene tree suggested at least one case of horizontal gene transfer. Overall, the results show that the thermophilic thiosulfate oxidizing isolates were adapted to life in presence of mercury mostly, but not exclusively, by possessing MerA. These findings suggest that reduction of mercury by chemolithotrophic thermophilic bacteria may mobilize mercury from sulfur and iron deposits in geothermal environments.

Comparative genomics of regulation of heavy metal resistance in Eubacteria

BACKGROUND

Heavy metal resistance (HMR) in Eubacteria is regulated by a variety of systems including transcription factors from the MerR family (COG0789). The HMR systems are characterized by the complex signal structure (strong palindrome within a 19 or 20 bp promoter spacer), and usually consist of transporter and regulator genes. Some HMR regulons also include detoxification systems. The number of sequenced bacterial genomes is constantly increasing and even though HMR resistance regulons of the COG0789 type usually consist of few genes per genome, the computational analysis may contribute to the understanding of the cellular systems of metal detoxification.

RESULTS

We studied the mercury (MerR), copper (CueR and HmrR), cadmium (CadR), lead (PbrR), and zinc (ZntR) resistance systems and demonstrated that combining protein sequence analysis and analysis of DNA regulatory signals it was possible to distinguish metal-dependent members of COG0789, assign specificity towards particular metals to uncharacterized loci, and find new genes involved in the metal resistance, in particular, multicopper oxidase and copper chaperones, candidate cytochromes from the copper regulon, new cadmium transporters and, possibly, glutathione-S-transferases.

CONCLUSION

Our data indicate that the specificity of the COG0789 systems can be determined combining phylogenetic analysis and identification of DNA regulatory sites. Taking into account signal structure, we can adequately identify genes that are activated using the DNA bending-unbending mechanism. In the case of regulon members that do not reside in single loci, analysis of potential regulatory sites could be crucial for the correct annotation and prediction of the specificity.

Mercury tolerance of thermophilic Bacillus sp. and Ureibacillus sp

Although resistance of microorganisms to Hg(II) salts has been widely investigated and resistant strains have been reported from many eubacterial genera, there are few reports of mercuric ion resistance in extremophilic microorganisms. Moderately thermophilic mercury resistant bacteria were selected by growth at 62 degrees C on Luria agar containing HgCl(2). Sequence analysis of 16S rRNA genes of two isolates showed the closest matches to be with Bacillus pallidus and Ureibacillus thermosphaericus. Minimum inhibitory concentration (MIC) values for HgCl(2) were 80 microg/ml and 30 microg/ml for these isolates, respectively, compared to 10 microg/ml for B. pallidus H12 DSM3670, a mercury-sensitive control. The best-characterised mercury-resistant Bacillus strain, B. cereus RC607, had an MIC of 60 microg/ml. The new isolates had negligible mercuric reductase activity but removed Hg from the medium by the formation of a black precipitate, identified as HgS by X-ray powder diffraction analysis. No volatile H(2)S was detected in the headspace of cultures in the absence or presence of Hg(2+), and it is suggested that a new mechanism of Hg tolerance, based on the production of non-volatile thiol species, may have potential for decontamination of solutions containing Hg(2+) without production of toxic volatile H(2)S.

Linkage of a novel mercury resistance operon with streptomycin resistance on a conjugative plasmid in Enterococcus faecium

It has been shown that the mercury in dental amalgam and other environmental sources can select for mercury resistant bacteria and that this can lead to an increase in resistance to antibiotics. To understand more about this linkage we have investigated the genetic basis for mercury and antibiotic resistance in a variety of oral bacteria. In this study we have cloned and sequenced the mer operon from an Enterococcus faecium strain which was resistant to mercury, tetracycline, and streptomycin. This strain was isolated, in a previous investigation, from a cynomolgus monkey post-installation of amalgam fillings. The mer operon was contained within a putative transposon (Tnmer1) of the ISL3 family. This element was located on a streptomycin resistant plasmid, pPPM1000, which shares homology with pRE25.

Diversity of mercury resistance determinants among Bacillus strains isolated from sediment of Minamata Bay

Thirty mercury-resistant (Hg R) Bacillus strains were isolated from mercury-polluted sediment of Minamata Bay, Japan. Mercury resistance phenotypes were classified into broad-spectrum (resistant to inorganic Hg(2+) and organomercurials) and narrow-spectrum (resistant to inorganic Hg(2+) and sensitive to organomercurials) groups. Polymerase chain reaction (PCR) product sizes and the restriction nuclease site maps of mer operon regions from all broad-spectrum Hg R Bacillus were identical to that of Bacillus megaterium MB1. On the other hand, the PCR products of the targeted merP (extracellular mercury-binding protein gene) and merA (intracellular mercury reductase protein gene) regions from the narrow-spectrum Hg R Bacillus were generally smaller than those of the B. megaterium MB1 mer determinant. Diversity of gene structure configurations was also observed by restriction fragment length polymorphism (RFLP) profiles of the merA PCR products from the narrow-spectrum Hg R Bacillus. The genetic diversity of narrow-spectrum mer operons was greater than that of broad-spectrum ones.

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.

The MerR family of transcriptional regulators

The MerR family is a group of transcriptional activators with similar N-terminal helix-turn-helix DNA binding regions and C-terminal effector binding regions that are specific to the effector recognised. The signature of the family is amino acid similarity in the first 100 amino acids, including a helix-turn-helix motif followed by a coiled-coil region. With increasing recognition of members of this class over the last decade, particularly with the advent of rapid bacterial genome sequencing, MerR-like regulators have been found in a wide range of bacterial genera, but not yet in archaea or eukaryotes. The few MerR-like regulators that have been studied experimentally have been shown to activate suboptimal sigma(70)-dependent promoters, in which the spacing between the -35 and -10 elements recognised by the sigma factor is greater than the optimal 17+/-1 bp. Activation of transcription is through protein-dependent DNA distortion. The majority of regulators in the family respond to environmental stimuli, such as oxidative stress, heavy metals or antibiotics. A subgroup of the family activates transcription in response to metal ions. This subgroup shows sequence similarity in the C-terminal effector binding region as well as in the N-terminal region, but it is not yet clear how metal discrimination occurs. This subgroup of MerR family regulators includes MerR itself and may have evolved to generate a variety of specific metal-responsive regulators by fine-tuning the sites of metal recognition.

Characterization of two regulatory genes of the mercury resistance determinants from TnMERI1 by luciferase-based examination

The broad-spectrum mercury resistance transposon, TnMERI1, of Bacillus megaterium strain MB1, contains three proposed operator/promoter (O/P) transcriptional start sites and two regulatory genes (merR1 and merR2). A series of luciferase (lux)-based transcriptional fusion plasmids were studied in Escherichia coli to show that both merR1 and merR2 gene products repressed transcription from O/PmerB3, O/PmerR1, and O/PmerR2 under uninduced conditions. Derepression occurred when the merR1 gene was present and Hg(2+) functioned as an inducer. In the presence of organomercurial compounds, basal transcription of merB3 was needed to produce inorganic Hg(2+) as the inducer of expression regulated by MerR1 at O/PmerB3. The presence of merR2 repressed transcription from all three O/Pmer sites under both non-induced conditions and when inorganic Hg(2+) or organomercurials were added. These results show that MerR1 functions as a repressor in the absence of Hg(2+) and as an activator in the presence of Hg(2+), while MerR2 functions as a repressor.

Thioredoxin reductase two modes of catalysis have evolved

Thioredoxin reductase (EC 1.6.4.5) is a widely distributed flavoprotein that catalyzes the NADPH-dependent reduction of thioredoxin. Thioredoxin plays several key roles in maintaining the redox environment of the cell. Like all members of the enzyme family that includes lipoamide dehydrogenase, glutathione reductase and mercuric reductase, thioredoxin reductase contains a redox active disulfide adjacent to the flavin ring. Evolution has produced two forms of thioredoxin reductase, a protein in prokaryotes, archaea and lower eukaryotes having a Mr of 35 000, and a protein in higher eukaryotes having a Mr of 55 000. Reducing equivalents are transferred from the apolar flavin binding site to the protein substrate by distinct mechanisms in the two forms of thioredoxin reductase. In the low Mr enzyme, interconversion between two conformations occurs twice in each catalytic cycle. After reduction of the disulfide by the flavin, the pyridine nucleotide domain must rotate with respect to the flavin domain in order to expose the nascent dithiol for reaction with thioredoxin; this motion repositions the pyridine ring adjacent to the flavin ring. In the high Mr enzyme, a third redox active group shuttles the reducing equivalent from the apolar active site to the protein surface. This group is a second redox active disulfide in thioredoxin reductase from Plasmodium falciparum and a selenenylsulfide in the mammalian enzyme. P. falciparum is the major causative agent of malaria and it is hoped that the chemical difference between the two high Mr forms may be exploited for drug design.

Analysis of a ferric leghemoglobin reductase from cowpea (Vigna unguiculata) root nodules

Ferric leghemoglobin reductase (FLbR), an enzyme reducing ferric leghemoglobin (Lb) to ferrous Lb, was purified from cowpea (Vigna unguiculata) root nodules by sequential chromatography on hydroxylapatite followed by Mono-Q HR5/5 FPLC and Sephacryl S-200 gel filtration. The purified cowpea FLbR had a specific activity of 216 nmol Lb(2+)O(2) formed min(-1) mg(-1) of enzyme for cowpea Lb(3+) and a specific activity of 184 nmol Lb(2+)O(2) formed min(-1) mg(-1) of enzyme for soybean Lb(3+). A cDNA clone of cowpea FLbR was obtained by screening a cowpea root nodule cDNA library. The nucleotide sequence of cowpea FLbR cDNA exhibited about 88% similarity with soybean (Glycine max) FLbR and 85% with pea (Pisum sativum) dihydrolipoamide dehydrogenase (DLDH, EC 1.8.1.4) cDNAs. Conserved regions for the FAD-binding site, NAD(P)H-binding site, and disulfide active site were identified among the deduced amino acid sequences of cowpea FLbR, soybean FLbR, pea DLDH and other enzymes in the family of the pyridine nucleotide-disulfide oxido-reductases.

Identification of three merB genes and characterization of a broad-spectrum mercury resistance module encoded by a class II transposon of Bacillus megaterium strain MB1

The complete structure of a broad-spectrum mercury resistance module was shown by sequencing the Gram-positive bacterial transposon TnMERI1 of Bacillus megaterium MB1. The regions encoding organomercury resistance were identified. Upstream of a previously identified organomercurial lyase merB (merB1) region of TnMERI1, a second merR (merR2) and a second merB gene (merB2) were found. These genes constitute a second operon (mer operon 2) following a promoter/operator (P(merR2)) region. A third organomercurial lyase gene (merB3) was found immediately upstream of the mer operon (mer operon 1) followed by a promoter/operator (P(merB3)) region homologous to that of the mer operon 1 (P(merR1)-merR1-merE-like-merT-merP-merA). The complete genetic structure of the mercury resistance module is organized as P(merB3)-merB3-P(merR1)-merR1-merE-like-merT+ ++ -merP-merA-P(merR2)-merR2 -merB2-merB1. The subcloning analysis of these three merB genes showed distinct substrate specificity as different organomercury lyase genes.

Mercury resistance in Bacillus cereus RC607: transcriptional organization and two new open reading frames

The chromosomal mercury resistance determinant of Bacillus cereus RC607 confers resistance to inorganic mercury and to organomercurials. The order of genes in the completed mercury resistance determinant is operator-promoter 1 (O/P1) merR1 merT open reading frame 3 (ORF3) ORF4 merA O/P2 merR2 merB2 merB1. The previously undetermined 1-kb DNA sequence between the merA and merB1 genes includes two significant ORFs, whose predicted protein products are homologous with MerR (the transcriptional regulator) and MerB (the organomercurial lyase enzyme). Two transcriptional start sites (promoters), O/P1 at the beginning of the determinant and O/P2 immediately upstream of the sixth ORF, the newly identified merR2, were mapped by reverse transcriptase (RT) primer extension. A long 6.3-kb mRNA traversing all eight ORFs was shown by RT-PCR. Growth sensitivity measurements in liquid media and cellular mercury volatization assays characterized inducibility and differences in functional activity in B. cereus RC607 and after cloning of the mer determinant into plasmids in Escherichia coli.

Structure analysis of a class II transposon encoding the mercury resistance of the Gram-positive Bacterium bacillus megaterium MB1, a strain isolated from minamata bay, Japan

A unique transposon was found in the chromosome of Bacillus megaterium MB1, a Gram-positive bacterium isolated from mercury-polluted sediments of Minamata Bay, Japan. The transposon region of a 14.5kb DNA fragment was amplified by PCR using a single PCR primer designed from the nucleotide sequence of an inverted repeat of class II transposons. The molecular analysis revealed that the PCR-amplified DNA fragment encodes a transposition module similar to that of Tn21. The transposon also encodes a broad-spectrum mercury resistance region having a restriction endonuclease map identical to that of Bacillus cereus RC607, a strain isolated from Boston Harbor, USA. The result of a phylogenetic analysis of the amino acid sequence of putative resolvase of the transposon showed that the transposon is phylogenetically closer to the transposons of Gram-positive bacteria than those of Gram-negative bacteria. Besides the transposition module and mer operon, the transposon encodes a mobile genetic element of bacterial group II introns between the resolvase gene and mer operon. The intron, however, does not intervene in any exon gene. The discovery of this newly found combination of the complex mobile elements may offer a clue to understanding the horizontal dissemination of broad-spectrum mercury resistance among microbes.

The merG gene product is involved in phenylmercury resistance in Pseudomonas strain K-62

The physiological function of a new gene, hereby designated merG, located between merA and merB on the broad-spectrum mer operon of Pseudomonas strain K-62 plasmid pMR26 was investigated. The 654-bp merG gene encodes a protein with a canonical leader sequence at its N terminus. The processing of the signal peptide of this protein was dose-dependently inhibited by sodium azide, a potent inhibitor of protein export. These results suggest that the mature MerG protein (ca. 20 kDa) may be located in the periplasm. Deletion of the merG gene from the broad-spectrum mer operon of pMR26 had no effect on the inorganic mercury resistance phenotype, but rendered the bacterium more sensitive to phenylmercury than its isogenic wild-type strain. Escherichia coli cells bearing pMU29, which carries a deletion of the merG gene, took up significantly more phenylmercury than the bacteria with the intact plasmid pMRA17. When the merG gene in a compatible plasmid was transformed into the E. coli strain carrying pMU29, the high uptake of and high sensitivity to phenylmercury were almost completely restored to their original levels. These results demonstrate that the merG gene is involved in phenylmercury resistance, presumably by reducing in-cell permeability to phenylmercury.

Mercury resistance is encoded by transferable giant linear plasmids in two chesapeake bay Streptomyces strains

The Streptomyces strains CHR3 and CHR28, isolated from the Baltimore Inner Harbor, contained two and one, respectively, giant linear plasmids which carry terminally bound proteins. The plasmids pRJ3L (322 kb), from CHR3, and pRJ28 (330 kb), from CHR28, carry genes homologous to the previously characterized chromosomal Streptomyces lividans 66 operon encoding resistance against mercuric compounds. Both plasmids are transmissible (without any detectable rearrangement) to the chloramphenicol-resistant S. lividans TK24 strain lacking plasmids and carrying a chromosomal deletion of the mer operon. S. lividans TK24 conjugants harboring pRJ3L or pRJ28 exhibited profiles of mercury resistance to mercuric compounds similar to those of Streptomyces strains CHR3 and CHR28.

Mercury-resistant actinomycetes from the Chesapeake Bay

Two actinomycete strains, CHR3 and CHR28, were isolated from metal-contaminated sediments from Baltimore Inner Harbor. The isolates were classified as Streptomyces spp. Agar diffusion assays showed both isolates to be resistant to mercuric chloride and phenylmercuric acetate. Hybridization experiments indicated that genes homologous to the mercuric reductase and organomercurial lyase of Streptomyces lividans 1326 were present in strains CHR3 and CHR28. Strain CHR28 grew at both low and high salt concentrations; however, strain CHR3 showed enhanced growth in the presence of salt, evidence of its habitat being marine or estuarine sediment.

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.

Missense mutation in flavin-containing mono-oxygenase 3 gene, FMO3, underlies fish-odour syndrome

Individuals with primary trimethylaminuria exhibit a body odour reminiscent of rotting fish, due to excessive excretion of trimethylamine (TMA; refs 1-3). The disorder, colloquially known as fish-odour syndrome, is inherited recessively as a defect in hepatic N-oxidation of dietary-derived TMA and cannot be considered benign, as sufferers may display a variety of psychosocial reactions, ranging from social isolation of clinical depression and attempted suicide. TMA oxidation is catalyzed by flavin-containing mono-oxygenase (FMO; refs 7,8), and tissue localization and functional studies have established FMO3 as the form most likely to be defective in fish-odour syndrome. Direct sequencing of the coding exons of FMO3 amplified from a patient with fish-odour syndrome identified two missense mutations. Although one of these represented a common polymorphism, the other, a C-->T transition in exon 4, was found only in an affected pedigree, in which it segregated with the disorder. The latter mutation predicts a proline-->leucine substitution at residue 153 and abolishes FMO3 catalytic activity. Our results indicate that defects in FMO3 underlie fish-odour syndrome and that the Pro 153-->Leu 153 mutation described here is a cause of this distressing condition.

Characterization of a gene encoding dihydrolipoamide dehydrogenase of the cyanobacterium Synechocystis sp. strain PCC 6803

The authors previously reported the isolation and partial characterization of a periplasmically located dihydrolipoamide dehydrogenase (LPD) from the cyanobacterium Synechocystis sp. strain PCC 6803. In the present work the gene (lpdA; database accession number Z48564) encoding the apoprotein of this LPD in Synechocystis PCC 6803 has been identified, sequenced and analysed. The lpdA gene codes for a protein starting with methionine, which is post-translationally removed. The mature protein contains an N-terminal serine and consists of 473 amino acids with a deduced molecular mass of 51421 Da (including one FAD). The LPD is an acidic protein with a calculated isoelectric point of 5.17. Comparison of the amino acid sequence of the Synechocystis LPD with protein sequences in the databases revealed that the enzyme shares identities of 31-35% with all 18 LPDs so far sequenced and published. As a first step in determining the role of this cyanobacterial LPD, attempts were made to generate an LPD-free Synechocystis mutant by insertionally inactivating the lpdA gene with a kanamycin-resistance cassette. However, the selected transformants appeared to be heteroallelic, containing both the intact lpdA gene and the lpdA gene inactivated by the drug-resistance cassette. The heteroallelic mutant studied, which had about 50% of the wild-type LPD activity, caused acidification of the growth medium. Growth over a prolonged time was only possible after an increased buffering of the medium. Since it is reported in the literature that inactivation of the pyruvate dehydrogenase complex (PDC) leads to acidosis, a function of the LPD in a cytoplasmic-membrane-associated PDC is conceivable.

Disruption of the Pseudomonas aeruginosa dipZ gene, encoding a putative protein-disulfide reductase, leads to partial pleiotropic deficiency in c-type cytochrome biogenesis

The Pseudomonas aeruginosa dipZ gene has been cloned and sequenced. Whereas disruption of Escherichia coli dipZ (dsbD), the hydrophilic C-terminal domain of which has been deduced to be periplasmic and to function as a protein-disulfide reductase, leads to the absence of c-type cytochromes, disruption of P. aeruginosa dipZ attenuated, but did not abolish, holo-c-type cytochrome biosynthesis. Comparison of the P. aeruginosa DipZ sequence with three other DipZ sequences indicated that there are not only two conserved cysteine residues in the C-terminal hydrophilic domain, but also two more in the central highly hydrophobic domain. The latter would be located toward the centre of two of the eight membrane-spanning alpha-helices predicted to compose the hydrophobic central domain of DipZ. Both these cysteine residues, plus other transmembrane helix residues, notably prolines and glycines, are also conserved in a group of membrane proteins, related to Bacillus subtilis CcdA, which lack the N- and C-terminal hydrophilic domains of the DipZ proteins. It is proposed that DipZ of P. aeruginosa and other organisms transfers reducing power from the cytoplasm to the periplasm through reduction and reoxidation of an intramembrane disulfide bond, or other mechanism involving these cysteine residues, and that this function can also be performed by B. subtilis CcdA and other CcdA-like proteins. The failure of dipZ disruption to abolish c-type cytochrome synthesis in P. aeruginosa suggests that, in contrast to the situation in E. coli, the absence of DipZ can be compensated for by one or more other proteins, for example a CcdA-like protein acting in tandem with one or more thioredoxin-like proteins.

Enterococcal transposon Tn5384: evolution of a composite transposon through cointegration of enterococcal and staphylococcal plasmids

Mechanisms for the possible transfer of antimicrobial resistance genes between staphylococci and enterococci remain poorly defined. We have previously reported the transfer between Enterococcus faecalis strains of a multiresistance chromosomal element (beta-lactamase positive and resistance to erythromycin, gentamicin, mercuric chloride, streptomycin, and tetracycline) which we have tentatively designated Tn5385. Tn5385 is a composite of several smaller transposable elements, including Tn5384, a 26-kb composite transposon conferring resistance to erythromycin, gentamicin, and mercuric chloride. Analyses of 7 kb within Tn5384 and flanking sequences within the larger element revealed sequences characteristic of staphylococcal beta-lactamase and small, mobilizable plasmids flanking a region with a sequence identical to those of the replication genes previously described for enterococcal and streptococcal broad-host-range plasmids. These diverse regions are linked by insertion sequences IS256 and IS257 in a manner which suggests a series of cointegration events as the genesis of the current relationship. Taken together, these data suggest that Tn5384 and the larger element within which it is incorporated (Tn5385) evolved at least in part as a result of cointegration between an enterococcal broad-host-range plasmid and staphylococcal beta-lactamase and small mobilizable plasmids. These results implicate broad-host-range plasmids in the transfer of resistance determinants from staphylococci to enterococci.

Isolation, partial characterization and localization of a dihydrolipoamide dehydrogenase from the cyanobacterium Synechocystis PCC 6803

A dihydrolipoamide dehydrogenase (LPD; dihydrolipoamide:NAD oxidoreductase, EC 1.8.1.4.) activity has been detected in the cyanobacterium Synechocystis PCC 6803. The enzyme was isolated from the membraneous fraction after detergent solubilization and shown to be homogenous on the basis of SDS-PAGE and N-terminal sequencing. The isolated enzyme had a specific activity of 75 U (mg protein)(-1) and was shown to be a homodimer with an apparent molecular mass of 104 kDa for the dimer and 55 kDa for the subunits. The enzyme contains 1.75 mol noncovalently bound FAD (mol enzyme)(-1) suggesting that each subunit contains 1 mol FAD and that the FAD is fairly tightly associated with the enzyme. N-terminal sequencing gave a contiguous amino acid sequence of 17 residues and showed that the N-terminus of the LPD from Synechocystis PCC 6803 has significant homologies to other LPDs sequenced so far. Immunoblot experiments indicated that the enzyme is mainly present in the membrane fraction, and immunocytochemical investigations gave evidence that the LPD in Synechocystis PCC 6803 is located in the periplasma space between the cytoplasma membrane and the peptidoglycan layer. This is the first report on an extracellular, membrane-bound LPD in a cyanobacterium.

Nucleotide sequence and expression of the organomercurial-resistance determinants from a Pseudomonas K-62 plasmid pMR26

pMRA17 cloned from Pseudomonas K-62 plasmid pMR26 specified the resistance to both organic and inorganic mercurials. DNA sequence of this broad-spectrum resistant mer operon was determined. The 5504-bp sequence includes six open reading frames (ORFs), five of which were identified as merR, merT, merP, merA and merB in order by analysis of deletion mutants and by comparison with the DNA and amino acid (aa) sequences of previously sequenced mer operons. The merB encoding organomercurial lyase showed a less identity than the other mer genes with those from other broad-spectrum resistance operons. The remaining ORF named merE, located between merA and merB, had no significant homology with the published mer genes and seemed to be a new gene which may involve in phenylmercury resistance. Induction experiments and maxicell analyses of the mer-polypeptides revealed that pMRA17 mer operon expressed mercurial-inducible phenotype and the merB and merE as well as the merA were under the control of MerR which could activate not only by mercuric ion but also by organomercurials.

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.

The mercury resistance operon of the IncJ plasmid pMERPH exhibits structural and regulatory divergence from other Gram-negative mer operons

The bacterial mercury resistance determinant carried on the IncJ plasmid pMERPH has been characterized further by DNA sequence analysis. From the sequence of a 4097 bp Bg/II fragment which confers mercury resistance, it is predicted that the determinant consists of the genes merT, merP, merC and merA. The level of DNA sequence similarity between these genes and those of the mer determinant of Tn21 was between 56 center dot 4 and 62 center dot 4%. A neighbour-joining phylogenetic tree of merA gene sequences was constructed which suggested that pMERPH bears the most divergent Gram-negative mer determinant characterized to date. Although the determinant from pMERPH has been shown to be inducible, no regulatory genes have been found within the Bg/II fragment and it is suggested that a regulatory gene may be located elsewhere on the plasmid. The cloned determinant has been shown to express mercury resistance constitutively. Analysis of the pMERPH mer operator/promoter (O/P) region in vivo has shown constitutive expression from the mer PTCPA promoter, which could be partially repressed by the presence of a trans-acting MerR protein from a Tn21-like mer determinant. This incomplete repression of mer PTCPA promoter activity may be due to the presence of an extra base between the -35 and -10 sequences of the promoter and/or to variation in the MerR binding sites in the O/P region. Expression from the partially repressed mer PTCPA promoter could be restored by the addition of inducing levels of Hg2+ ions. Using the polymerase chain reaction with primers designed to amplify regions in the merP and merA genes, 1 center dot 37 kb pMERPH-like sequences have been amplified from the IncJ plasmid R391, the environmental isolate SE2 and from DNA isolated directly from non-cultivated bacteria in River Mersey sediment. This suggests that pMERPH-like sequences, although rare, are nevertheless persistent in natural environments.

Insertional inactivation of a chromosomal locus that modulates expression of potential virulence determinants in Staphylococcus aureus

A single insertion of transposon Tn551 into a unique chromosomal locus of Staphylococcus aureus ISP479C has resulted in a pleiotropic effect on the expression of both extracellular and cell wall proteins. In particular, the expression of cell wall protein A and clumping activity with fibrinogen were rendered undetectable in the mutant 1E3 compared with the parent. The secretion of alpha-hemolysin in mutant 1E3 was modestly increased. Southern blot and phenotypic analyses indicated that this locus is distinct from agr, xpr, and sar, three previously described global regulatory loci. Transduction experiments demonstrated that the genotype associated with mutant 1E3 could be transferred back into the parental strain ISP479C. The transductant 1E3-2 displayed a phenotypic profile similar to that of the original mutant. Northern (RNA) blot studies showed that this locus may be involved in modulating target genes at the mRNA level. In the rabbit endocarditis model, there was a significant decrease in both the infectivity rate and intravegetation bacterial density with mutant 1E3 compared with the parent at an inoculum of 10(3) CFU. Since protein A and the fibrinogen-binding protein(s) are major surface proteins that may mediate bacterial adhesion to host tissues, this locus may be an important genetic element involved in the expression of virulence determinants in S. aureus.

An L40C mutation converts the cysteine-sulfenic acid redox center in enterococcal NADH peroxidase to a disulfide

Multiple sequence alignments including the enterococcal NADH peroxidase and NADH oxidase indicate that residues Ser38 and Cys42 align with the two cysteines of the redox-active disulfides found in glutathione reductase (GR), lipoamide dehydrogenase, mercuric reductase, and trypanothione reductase. In order to evaluate those structural determinants involved in the selection of the cysteine-sulfenic acid (Cys-SOH) redox centers found in the two peroxide reductases and the redox-active disulfides present in the GR class of disulfide reductases, NADH peroxidase residues Ser38, Phe39, Leu40, and Ser41 have been individually replaced with Cys. Both the F39C and L40C mutant peroxidases yield active-site disulfides involving the new Cys and the native Cys42; formation of the Cys39-Cys42 disulfide, however, precludes binding of the FAD coenzyme. In contrast, the L40C mutant contains tightly-bound FAD and has been analyzed by both kinetic and spectroscopic approaches. In addition, the L40C and S41C mutant structures have been determined at 2.1 and 2.0 A resolution, respectively, by X-ray crystallography. Formation of the Cys40-Cys42 disulfide bond requires a movement of Cys42-SG to a new position 5.9 A from the flavin-C(4a) position; this is consistent with the inability of the new disulfide to function as a redox center in concert with the flavin. Stereochemical constraints prohibit formation of the Cys41-Cys42 disulfide in the latter mutant.

Organomercurial resistance determinants in Pseudomonas K-62 are present on two plasmids

Pseudomonas strain K-62 was found to contain six plasmids. A mutant derivative cured of the 26-kb plasmid showed a higher sensitivity to mercurials; however, the strain was still able to volatilize them. Loss of the 68-kb plasmid in addition to the 26-kb plasmid abolished the ability of mercury volatilization in this strain and led to a further decrease in the level of mercurial resistance. These results are the first to demonstrate that the organomercurial resistance of Pseudomonas strain K-62 is plasmid-based, and that both the 26- and 68-kb plasmids are required for full expression of the mercurial resistance. Probes specific for the mer genes merA, merB, and merR strongly hybridized with the 26-kb plasmid, but not with the 68-kb plasmid. Two fragments of the 26-kb plasmid that hybridized with the mer genes were cloned and expressed in Escherichia coli. One recombinant plasmid (pMRA17) inducibly encoded a typical broad-spectrum mercurial resistance, whereas the other recombinant plasmid (pMRB01) constitutively conferred hypersensitivity to phenylmercury in the absence of mercuric reductase activity. The results suggest that the two organomercurial lyases in the cells are transcribed from different operator-promoters.

Sequence, mapping and disruption of CCC2, a gene that cross-complements the Ca(2+)-sensitive phenotype of csg1 mutants and encodes a P-type ATPase belonging to the Cu(2+)-ATPase subfamily

We have isolated, sequenced, mapped and disrupted a gene, CCC2, from Saccharomyces cerevisiae. This gene displays non-allelic complementation of the Ca(2+)-sensitive phenotype conferred by the csg1 mutation. Analysis of the CCC2p amino acid sequence reveals that it encodes a member of the P-type ATPase family and is most similar to a subfamily thought to consist of Cu2+ transporters, including the human genes that mutate to cause Wilson disease and Menkes disease. The ability of this gene, in two or more copies, to reverse the csg1 defect suggests that Ca(2+)-induced death of csg1 mutant cells is related to Cu2+ metabolism. Cells without CCC2 require increased Cu2+ concentrations for growth. Therefore CCC2p may function to provide Cu2+ to a cellular compartment rather than in removal of excess Cu2+.

Tributyltin-resistant marine bacteria: a summary of recent work

A tributyltin chloride (TBTCl)-resistant bacterium, Alteromonas sp. M-1, was isolated from coastal seawater. This bacterium grew in medium containing 125 microM TBTCl. TBTCl added to the medium was taken up by this bacterium, however, the amount of TBTCl in the cellular fraction was low after the logarithmic phase, suggesting the existence of a TBTCl-efflux system. A genetic library was constructed using plasmid vector pUC 19. Three positive clones were obtained, by which E. coli was transformed to TBTCl resistance. Of the three clones, the shortest fragment from HindIII-library was analyzed. This fragment was 1.8 kb long and contained one complete open reading frame. The predicted amino acid sequence of this open reading frame had a homologous domain to transglycosylases of bacteriophage and E. coli. TBTCl-tolerant marine bacteria other than Alteromonas sp. M-1 were obtained from natural seawater to which TBTCl was added. DNA-DNA hybridization was performed between the three cloned fragments from Alteromonas sp. M-1 and chromosomal DNA of the TBTCl-tolerant bacteria. Some strains hybridized with the fragments and some did not, suggesting that several genes are responsible for TBTCl tolerance.

Bacterial resistance mechanisms for heavy metals of environmental concern

Bacterial species have genetically-determined systems for resistances to toxic heavy metals. Those for metals of environmental concern including mercury cadmium, arsenic and others are briefly summarized, considering the genes of the systems and the biochemical mechanisms by which the resistance proteins function.

Molecular analysis of mercury-resistant Bacillus isolates from sediment of Minamata Bay, Japan

Bacillus isolates from Minamata Bay sediment were selected for the ability to volatilize mercury from a range of organomercurials (including methylmercury). Chromosomal DNA from 74 of 78 such strains hybridized with the mer mercury resistance operon DNA from marine Bacillus sp. strain RC607 (Y. Wang, M. Moore, H. S. Levinson, S. Silver, C. Walsh, and I. Mahler, J. Bacteriol. 171:83-92, 1989). The most frequent classes with regard to restriction nuclease site maps of the mer operon for the new isolates were identical to or closely related to the mer determinant of strain RC607. PCR amplification analysis with primers designed from the strain RC607 mer operon gave products of precisely the predicted size with the 74 Minamata Bay isolates.