Location on RP4 of a tellurite resistance determinant not normally expressed in IncP alpha plasmids

Tellurite resistance and reduction by obligately aerobic photosynthetic bacteria

Seven species of obligately aerobic photosynthetic bacteria of the genera Erythromicrobium, Erythrobacter, and Roseococcus demonstrated high-level resistance to tellurite and accumulation of metallic tellurium crystals. High-level resistance without tellurite reduction was observed for Roseococcus thiosulfatophilus and Erythromicrobium ezovicum grown with certain organic carbon sources, implying that tellurite reduction is not essential to confer tellurite resistance.

The bacterial response to the chalcogen metalloids Se and Te

Microbial metabolism of inorganics has been the subject of interest since the 1970s when it was recognized that bacteria are involved in the transformation of metal compounds in the environment. This area of research is generally referred to as bioinorganic chemistry or microbial biogeochemistry. Here, we overview the way the chalcogen metalloids Se and Te interact with bacteria. As a topic of considerable interest for basic and applied research, bacterial processing of tellurium and selenium oxyanions has been reviewed a few times over the past 15 years. Oddly, this is the first time these compounds have been considered together and their similarities and differences highlighted. Another aspect touched on for the first time by this review is the bacterial response in cell-cell or cell-surface aggregates (biofilms) against the metalloid oxyanions. Finally, in this review we have attempted to rationalize the considerable amount of literature available on bacterial resistance to the toxic metalloids tellurite and selenite.

Glutathione is a target in tellurite toxicity and is protected by tellurite resistance determinants inEscherichia coli

Tellurite (TeO32-) is highly toxic to most microorganisms. The mechanisms of toxicity or resistance are poorly understood. It has been shown that tellurite rapidly depletes the reduced thiol content within wild-type Escherichia coli. We have shown that the presence of plasmid-borne tellurite-resistance determinants protects against general thiol oxidation by tellurite. In the present study we observe that the tellurite-dependent depletion of cellular thiols in mutants of the glutathione and thioredoxin thiol:redox system was less than in wild-type cells. To identify the type of low-molecular-weight thiol compounds affected by tellurite exposure, the thiol-containing molecules were analyzed by reverse phase HPLC as their monobromobimane derivatives. Results indicated that reduced glutathione is a major initial target of tellurite reactivity within the cell. Other thiol species are also targeted by tellurite, including reduced coenzyme A. The presence of the tellurite resistance determinants kilA and ter protect against the loss of reduced glutathione by as much as 60% over a 2 h exposure. This protection of glutathione oxidation is likely key to the resistance mechanism of these determinants. Additionally, the thiol oxidation response curves were compared between selenite and tellurite. The loss of thiol compounds within the cell recovered from selenite but not to tellurite.Key words: tellurite, resistance, thiol oxidation, heavy metal toxicity, selenite, glutathione.

Tellurite-mediated thiol oxidation in Escherichia coli

The oxyanion of tellurium, tellurite (TeO3(2-)), is toxic to most micro-organisms, particularly gram-negative bacteria. The mechanism of tellurite toxicity is presently unknown. Many heavy metals and oxyanions, including tellurite, interact with reduced thiols (RSH). To determine if tellurite interaction with RSH groups is involved in the toxicity mechanism, the RSH content of Escherichia coli cultures was assayed. After exposure to tellurite, cells were harvested and lysed in the presence of the RSH-specific reagent 5,5'-dithiobis(2-nitrobenzoic acid). Upon exposure of tellurite-susceptible cells to TeO3(2-), the RSH content decreased markedly. Resistance to potassium tellurite (Te(r)) in gram-negative bacteria is encoded by plasmids of incompatibility groups IncFI, IncP alpha, IncHI2, IncHI3 and IncHII, as well as the tehAtehB operon from the E. coli chromosome. When cells harbouring a Te(r) determinant were exposed to TeO3(2-), only a small fraction of the RSH content became oxidized. In addition to tellurite-dependent thiol oxidation, the resistance of E. coli mutants affected in proteins involved in disulfide-bond formation (dsb) was investigated. Mutant strains of dsbA and dsbB were found to be hypersensitive to tellurite (MIC 0.008-0.015 microg K2TeO3 ml(-1) compared to wild-type E. coli with MICs of 1-2 microg K2TeO3 ml(-1)). In contrast, dsbC and dsbD mutants showed no hypersensitivity. The results suggest that hypersensitivity to tellurite is reliant on the presence of an isomerase activity and not the thiol oxidase activity of the Dsb proteins. The results establish that the Te(r) determinants play an important role in maintaining homeostasis of the intracellular reducing environment within gram-negative cells through specific reactions with either TeO3(2-) or thiol:tellurium products.

The kilE locus of promiscuous IncP alpha plasmid RK2 is required for stable maintenance in Pseudomonas aeruginosa

Eight coordinately regulated operons constitute the kor regulon of the IncP alpha plasmid RK2. Three operons specify functions required for replication initiation, conjugative transfer, and control of gene expression. The functions of the other operons, including those of the four coregulated operons that compose the kilA, kilC, and kilE loci, have not been determined. Here, we present the first evidence that a kil determinant is involved in IncP plasmid maintenance. Elevation of KorC levels specifically to reduce the expression of the KorC-regulated kilC and kilE operons severely affected the maintenance of both the IncP alpha plasmid RK2lac and the IncP beta plasmid R751 in Pseudomonas aeruginosa but had little effect on plasmid maintenance in Escherichia coli. Precise deletion of the two kilE operons from RK2lac was achieved with the VEX mutagenesis system for large genomes. The resulting plasmid showed significant loss of stability in P. aeruginosa only. The defect could be complemented by reintroduction of kilE at a different position on the plasmid. The instability of the RK2lac delta kilE mutant did not result from a reduction in average plasmid copy number, reduced expression of kilC, decreased conjugative transfer, or loss of the korE regulator. We found that both the par and kilE loci are required for full stability of RK2lac in P. aeruginosa and that the par and kilE functions act independently. These results demonstrate a critical role for the kilE locus in the stable inheritance of RK2 in P. aeruginosa.

Cytological aspects of resistance to potassium tellurite conferred on Pseudomonas cells by plasmids

The ultrastructure of strains Pseudomonas putida BS228 and Pseudomonas aeruginosa ML4262 harboring plasmids pBS10, pBS31, and pBS221, which determine resistance to potassium tellurite, was studied. Bacteria were grown in media containing increasing concentrations of potassium tellurite. Crystalline structures containing tellurium appeared in their periplasmic space. The dynamics of crystal growth was studied. Crystals were released into the medium by pinching off of the outer membrane vesicles containing growing crystals. A possible mechanism of this process was described; cytobiochemical peculiarities were discussed.

Neither reduced uptake nor increased efflux is encoded by tellurite resistance determinants expressed in Escherichia coli

Rates of uptake of the TeO3(2-) oxyanion were investigated in Escherichia coli cells containing tellurite resistance determinants from both plasmid (RK2Ter, R478, pMER610, MIP233, pHH1508a, pMUR) and chromosomal (tehAB) sources. The uptake was investigated to determine whether or not reduced uptake or increased efflux is involved in the tellurite resistance mechanism. Reduced TeO3(2-) uptake generated by cultures harboring arsABC from the plasmid R773, which has been previously shown to be an oxyanion efflux transporter, was used as the standard. Uptake curves were found to be essentially identical among E. coli cultures harboring the tellurite resistance plasmids RK2Ter, pMER610, pHH1508a, and pMUR and cultures harboring tellurite-sensitive control plasmids. Cultures harboring clones of the tehAB operon from E. coli showed no change in the TeO3(2-) accumulation. Cultures harboring R478 demonstrated reduced uptake. However, a subclone containing only the tellurite resistance determinant displayed no reduced uptake. This suggests that there may be another determinant on R478 other than the primary tellurite resistance determinant that gives rise to TeO3(2-) efflux. These results demonstrate that neither reduced uptake nor increased efflux is responsible for the tellurite resistance in the resistance determinants investigated here.

Functional expression of the tellurite resistance determinant from the IncHI-2 plasmid pMER610

The transpositional phage MudI 1734 lacZ was used to construct transcriptional fusions within the plasmid pMJ611, which contains the cloned tellurite resistance (TeR) determinant of the IncHI-2 plasmid pMER610. A series of 70 MudI insertions, in both orientations, causing loss of tellurite resistance in pMJ611, mapped within a 4.3 kb region which included the genes terA-terD and a 0.4 kb region upstream of the site previously reported as the 5' limit of the TeR determinant. Expression of beta-galactosidase from these transcriptional fusions, including those involving the 5' upstream region, occurred only from inserts transcribed in the direction terA-terD, confirming the transcriptional orientation of the TeR determinant deduced from DNA sequence analysis. Sixteen of the tellurite-sensitive MudI fusions, distributed over the entire determinant and in both orientations, showed the same pattern of expression when transferred by conjugation and homologous recombination to pMER610, except that the beta-galactosidase levels were consistently 2- to 3-fold higher in the parent plasmid. Northern analysis with a DNA probe spanning the TeR determinant identified five transcripts of 4.8, 4.0, 2.7, 1.5 and 1.0 kb synthesised by pMER610. Further hybridisations with DNA probes defining sub-sections of the TeR determinant, together with DNA sequence analysis, suggested the presence of three transcriptional start sites, at approximately 0.9 and 0.1 kb upstream of terA, and near the junction between terC and terD. Three transcriptional termination sites, located within terA, near the terC-terD junction and at the 3' end of terE are also indicated. Both the expression of beta-galactosidase from the MudI fusions and the synthesis of ter gene transcripts are constitutive and were not affected by prior exposure of cultures to sub-toxic levels of tellurite. Further DNA sequence analysis reveals that the extensive homology between terD and terE extends to a section of terA.

Inhibition of bacteriophage lambda development by the klaA gene of broad-host-range plasmid RK2

The kil-kor regulon of broad-host-range plasmid RK2 is an unusual array of eight co-regulated operons that express at least 21 genes, including the plasmid replication initiator gene. Some of the operons were first identified as kil loci because uncontrolled expression in the absence of certain kor regulatory genes leads to death of the host cells. The functions of kilA, C and E are unknown, although co-regulation with the replication initiator gene suggests that they may have importance in the maintenance or host range of the plasmid. Here we report studies on the function of klaA, the first of three host-lethal genes in the kilA operon. We found that lambda pklaA-1, a lambda phage containing the klaA gene, is unable to form plaques unless the host expresses the KorA and KorB repressors needed to regulate transcription from the klaA promoter. The failure to form plaques depends on the klaA gene product and results from the inability of infected cells to produce viable phage particles. Transcription of early, delayed early and late genes or processing of lambda DNA are not affected by klaA overexpression, while cell lysis, lambda DNA replication and production of functional phage heads are reduced. However, the failure to produce viable phage is best explained by the inability to synthesize lambda tails. The finding that klaA strongly inhibits a specific morphogenetic step in the assembly of lambda phage particles has significance with respect to the function of klaA on plasmid RK2.

Plasmid-mediated resistance to tellurite: expressed and cryptic

The ability of some bacteria to grow in the presence of high concentrations of tellurium compounds has been recognized for almost 100 years. Since then, interest in this phenomenon has generated a slow but steady trickle of literature. In the past few years, the use of modern techniques in molecular biology has led to a dramatic increase in our understanding of the genetics of several bacterial determinants for resistance to tellurium compounds. These determinants are frequently found to be encoded by plasmids which carry multiple antibiotic resistance determinants. Our understanding of the biochemistry of these systems remains limited. In this article, the history of the study of bacterial resistance to tellurium compounds is briefly reviewed. This is followed by an analysis of the recent developments in the study of plasmid-mediated resistance determinants. Finally, preliminary investigations on the possible mechanisms of bacterial resistance to tellurium compounds are presented.

The kilA operon of promiscuous plasmid RK2: the use of a transducing phage (lambda pklaA-1) to determine the effects of the lethal klaA gene on Escherichia coli cells

The kil-kor regulon of promiscuous plasmid RK2 includes the replication initiator gene trfA and several potentially host-lethal kil loci (kilA, kilB, kilC, kilE), whose functions may be involved in plasmid maintenance or broad host range. The kilA locus consists of a single operon of three genes (klaA, klaB, klaC), each of which is lethal when expressed from the klaA promoter in the absence of repressors encoded by korA and korB. In this study, we examined the effects of the unregulated klaA gene on the host cell. Bacteriophage lambda was used to construct a transducing phage (lambda pklaA-1) that allows efficient introduction of the klaA gene into Escherichia coli. Cells lacking korA and korB (to allow uncontrolled expression of klaA) and expressing lambda repressor (to prevent phage lytic growth) are killed by lambda pklaA-1. Cell death is dependent on the klaA structural gene, independent of the SOS system of the host, and is prevented by the presence of korA and korB. lambda pklaA-1 was used to synchronously infect cells lacking korA and korB to determine the effects of klaA on the cells over time. The earliest effects, visible at two hours post-infection, are inhibition of growth of the culture, formation of elongated cells, and striking changes in the appearance of the outer membrane. After four to five hours, the viability of the culture declined sharply and macromolecular synthesis ceased. The distinct class of early events is consistent with the hypothesis that the KlaA polypeptide interacts with a specific target in the host cell.

Structural, molecular, and genetic analysis of the kilA operon of broad-host-range plasmid RK2

The kil loci (kilA, kilB, kilC, and kilE) of incompatibility group P (IncP), broad-host-range plasmid RK2 were originally detected by their potential lethality to Escherichia coli host cells. Expression of the kil determinants is controlled by different combinations of kor functions (korA, korB, korC, and korE). This system of regulated genes, known as the kil-kor regulon, includes trfA, which encodes the RK2 replication initiator. The functions of the kil loci are unknown, but their coregulation with an essential replication function suggests that they have a role in the maintenance or host range of RK2. In this study, we have determined the nucleotide sequence of a 3-kb segment of RK2 that encodes the entire kilA locus. The region encodes three genes, designated klaA, klaB, and klaC. The phage T7 RNA polymerase-dependent expression system was use to identify three polypeptide products. The estimated masses of klaA and klaB products were in reasonable agreement with the calculated molecular masses of 28,407 and 42,156 Da, respectively. The klaC product is calculated to be 32,380 Da, but the observed polypeptide exhibited an apparent mass of 28 kDa on sodium dodecyl sulfate-polyacrylamide gels. Mutants of klaC were used to confirm that initiation of translation of the observed product occurs at the first ATG in the klaC open reading frame. Hydrophobicity analysis indicated that the KlaA and KlaB polypeptides are likely to be soluble, whereas the KlaC polypeptide was predicted to have four potential membrane-spanning domains. The only recognizable promoter sequences in the kilA region were those of the kilA promoter located upstream of klaA and the promoter for the korA-korB operon located just downstream of a rho-independent terminatorlike sequence following klaC. The transcriptional start sites for these promoters were determined by primer extension. Using isogenic sets of plasmids with nonpolar mutations, we found that klaA, klaB, and klaC are each able to express a host-lethal (Kil+) phenotype in the absence of kor functions. Inactivation of the kilA promoter causes loss of the lethal phenotype, demonstrating that all three genes are expressed from the kilA promoter as a multicistronic operon. We investigated two other phenotypes that have been mapped to the kilA region of RK2 or the closely related IncP plasmids RP1 and RP4: inhibition of conjugal transfer of IncW plasmids (fwB) and resistance to potassium tellurite. The cloned kilA operon was found to express both phenotypes, even in the presence of korA and korB, whose functions are known to regulate the kilA promoter. In addition, mutant and complementation analyses showed that the kilA promoter and the products of all three kla genes are necessary for expression of both phenotypes. Therefore, host lethality, fertility inhibition, and tellurite resistance are all properties of the kilA operon. We discuss the possible role of the kilA operon for RK2.

Transcriptional analysis, translational analysis, and sequence of the kilA-tellurite resistance region of plasmid RK2Ter

The tellurite resistance (Ter) determinant of the IncP alpha plasmid RK2Ter, a variant of RK2 (also called RP4), is located between the kilA and korA genes involved in plasmid replication control. Transcriptional and translational fusions were constructed between the gene for beta-galactosidase and the kilA and Ter genes by using the transpositional phage mini-Mu. These fusions indicated that the Ter genes are transcribed in the same direction as kilA and that transcription and translation of the cloned kilA gene are occurring and may not be lethal to the bacterial cell even in the absence of korA. The nucleotide sequence of this region was determined, and three open reading frames (ORFs) were identified. The first ORF codes for KilA, a 28-kDa hydrophilic protein. The second ORF, telA, codes for a hydrophilic protein of 42 kDa. The third ORF, telB, codes for a hydrophobic protein of 32 kDa. This protein appears to be located in the inner membrane of the bacterial cell, since fusions of TelB to alkaline phosphatase were obtained by using TnphoA. All three proteins were detected by sodium dodecyl sulfate-polyacrylamide gel electrophoresis after overproduction using the T7 RNA polymerase/promoter system. The same three proteins were produced when Tes and Ter derivatives of RP4 were expressed in an in vitro transcription-translation system. A single Ser-to-Cys missense mutation in telB was found to be responsible for mutation of RK2 to Ter.

The kil-kor regulon of broad-host-range plasmid RK2: nucleotide sequence, polypeptide product, and expression of regulatory gene korC

Broad-host-range plasmid RK2 encodes several kil operons (kilA, kilB, kilC, kilE) whose expression is potentially lethal to Escherichia coli host cells. The kil operons and the RK2 replication initiator gene (trfA) are coregulated by various combinations of kor genes (korA, korB, korC, korE). This regulatory network is called the kil-kor regulon. Presented here are studies on the structure, product, and expression of korC. Genetic mapping revealed the precise location of korC in a region near transposon Tn1. We determined the nucleotide sequence of this region and identified the korC structural gene by analysis of korC mutants. Sequence analysis predicts the korC product to be a polypeptide of 85 amino acids with a molecular mass of 9,150 daltons. The KorC polypeptide was identified in vivo by expressing wild-type and mutant korC alleles from a bacteriophage T7 RNA polymerase-dependent promoter. The predicted structure of KorC polypeptide has a net positive charge and a helix-turn-helix region similar to those of known DNA-binding proteins. These properties are consistent with the repressorlike function of KorC protein, and we discuss the evidence that KorA and KorC proteins act as corepressors in the control of the kilC and kilE operons. Finally, we show that korC is expressed from the bla promoters within the upstream transposon Tn1, suggesting that insertion of Tn1 interrupted a plasmid operon that may have originally included korC and kilC.

Genetic analysis of transfer and incompatibility functions within the IncHI plasmid R27

This study was undertaken to establish a transfer complementation system for IncH plasmids and to locate regions of incompatibility within the HI1 plasmid, R27. Two regions of R27 were found to contribute to incompatibility as determined by incompatibility testing with fragments of R27 cloned in cosmid vectors. One of these regions hybridized with the IncHI1 rep probe (Couturier et al., Microbiol. Rev. 52, 375-395, 1988). Complementation analysis was carried out using transfer-deficient mutants of R27 in combination with pHH1508a. Cosmid vectors, which contained cloned restriction fragments of R27, were able to complement selected R27 Tra- mutants, enabling the transfer-deficient plasmid to transfer at near-normal frequencies. Complementation of R27 Tra- plasmids by pHH1508a at both 26 and 37 degrees C was shown to occur, but was host-dependent in its degree. These results suggest that the transfer mechanisms of IncHI and IncHII plasmids are related.

Comparison of tellurite resistance determinants from the IncP alpha plasmid RP4Ter and the IncHII plasmid pHH1508a

The tellurite resistance (Ter) determinants of the IncHII plasmid pHH1508a and the broad host range IncP alpha plasmid RP4Ter were cloned into pUC8, creating plasmids pDT1364 and pDT1558, respectively. The Ter region of pDT1364 was localized to a 1.25-kilobase region by using Tn1000 insertion mutagenesis. Insertions of Tn1000 into pDT1558 which resulted in tellurite sensitivity spanned 1.75 kilobases of DNA. No similarity between the restriction maps of these two plasmids was observed, and no homology could be detected by DNA-DNA hybridization. Expression in an in vitro transcription-translation system showed that pDT1364 encoded two polypeptides with molecular masses of 23 and 12 kilodaltons (kDa) which were not expressed by pUC8. Some of the Tn1000 insertion mutants did not express the 23-kDa protein. pDT1558 encoded a 40-kDa polypeptide which was not expressed by pUC8. Both Ter determinants were expressed constitutively. Our findings suggest that the mechanisms of Ter encoded by these two plasmids are different.

Purification and biochemical characterization of tellurite-reducing activities from Thermus thermophilus HB8

Cell-free extracts of Thermus thermophilus HB8 catalyze the in vitro, NADH-dependent reduction of potassium tellurite (K2TeO3). Three different protein fractions with tellurite-reducing activities were identified. Two exhibited high molecular weight and were composed of at least two different polypeptides. The protein in the third fraction was purified to homogeneity and had a single polypeptide chain of 53 to 54 kilodaltons, with an isoelectric point of 8.1. Each enzyme was thermostable, the temperature optimum was 75 degrees C, and 30 mM NaCl, 1.5 M urea, or 0.004% sodium dodecyl sulfate caused 50% inhibition of the enzymes. However, 2% Triton X-100 did not have an inhibitory effect. The enzymes were also able to catalyze the reduction of sodium selenite and sodium sulfite in vitro. NADH was replaceable by NADPH. Divalent cations, such as Ca2+ and Ba2+, had no effect on the activity, while similar concentrations of Zn2+, Ni2+, and Cu2+ abolished the activity. This reductase activity could enable these bacteria both to reduce K2TeO3 and to increase their tolerance toward this salt.

Structure and location of tellurium deposited in Escherichia coli cells harbouring tellurite resistance plasmids

The plasmids RP4Ter and pHH1508a, which belong to the P and HII incompatibility groups, respectively, confer resistance to potassium tellurite (K2TeO3) on Escherichia coli. The genes for tellurite resistance were cloned from each plasmid onto the vector pUC8 to create pDT1366 and pD1364, respectively. Unstained, unfixed bacteria carrying these plasmids contained black intracellular deposits when grown on media containing tellurite. Thin sections of these bacteria fixed with glutaraldehyde were prepared and examined by electron microscopy. The black deposits were located inside the cell and were frequently associated with the inner membrane of the bacterium. Bacteria containing pDT1366 or pDT1364, and therefore a higher gene dosage of the Ter determinant, contained more black deposits, but had a decreased resistance, as measured by the minimum inhibitory concentration using the agar dilution method. Using the technique of electron spectroscopic imaging, the black intracellular deposits were shown to contain predominantly reduced metallic tellurium, and significant amounts of oxygen or carbon, thereby confirming earlier results using X-ray diffraction analysis of whole cells.

Nucleotide sequence of korB, a replication control gene of broad host-range plasmid RK2

The korB gene is a major regulatory element in the replication and maintenance of broad host-range plasmid RK2. It negatively controls the replication gene trfA, the host-lethal determinants kilA and kilB, and the korA-korB operon. Here, we present the nucleotide sequence of an 1167 base-pair region that encodes korB. Using sequence data from korB mutants, we identified the korB structural gene. The predicted polypeptide product is negatively charged and has a molecular weight of 39,015, which is considerably less than that estimated by its electrophoretic mobility in SDS/polyacrylamide gels. Secondary-structure predictions of korB polypeptide revealed three closely spaced helix-turn-helix regions with significant homology to similar structures in known DNA-binding proteins. The korB gene, like all other sequenced RK2 genes, shows a strong preference for codons ending in a G or C residue. This is similar to codon usage by genes of Klebsiella and Pseudomonas, the original hosts for RK2 and some closely related plasmids. We also sequenced the site of transposon Tn76 insertion in the host-range mutant pRP761 and found it to be located immediately upstream from korB in the incC gene. Finally, we report the presence of sequences resembling a replication origin within the korB structural gene: a cluster of four 19 base-pair direct repeats and a nearby potential binding site for Escherichia coli dna A replication protein.