Restriction map of the antibiotic resistance plasmid R1drd-19 and its derivatives pKN102 (R1drd-19B2) and R1drd-16 for the enzymes BamHI, HindIII, EcoRI and SalI

Doxycycline induces Hok toxin killing in host E. coli

The antibacterial efficacy of the tetracycline antibiotics has been greatly reduced by the development of resistance, hence a decline in their clinical use. The hok/sok locus is a type I toxin/antitoxin plasmid stability element, often associated with multi-drug resistance plasmids, especially ESBL-encoding plasmids. It enhances host cell survivability and pathogenicity in stressful growth conditions, and increases bacterial tolerance to β-lactam antibiotics. The hok/sok locus forms dsRNA by RNA:RNA interactions between the toxin encoding mRNA and antitoxin non-coding RNA, and doxycycline has been reported to bind dsRNA structures and inhibit their cleavage/processing by the dsRNase, RNase III. This study investigated the antibacterial activities of doxycycline in hok/sok host bacteria cells, the effects on hok/sok-induced changes in growth and the mechanism(s) involved. Diverse strains of E. coli were transformed with hok/sok plasmids and assessed for doxycycline susceptibility and growth changes. The results show that the hok/sok locus increases bacterial susceptibility to doxycycline, which is more apparent in strains with more pronounced hok/sok-induced growth effects. The increased doxycycline susceptibility occurs despite β-lactam resistance imparted by hok/sok. Doxycycline was found to induce bacterial death in a manner phenotypically characteristic of Hok toxin expression, suggesting that it inhibits the toxin/antitoxin dsRNA degradation, leading to Hok toxin expression and cell death. In this way, doxycycline could counteract the multi-drug resistance plasmid maintenance/propagation, persistence and pathogenicity mechanisms associated with the hok/sok locus, which could potentially help in efforts to mitigate the rise of antimicrobial resistance.

Toxins targeting transfer RNAs: Translation inhibition by bacterial toxin-antitoxin systems

Prokaryotic toxin-antitoxin (TA) systems are composed of a protein toxin and its cognate antitoxin. These systems are abundant in bacteria and archaea and play an important role in growth regulation. During favorable growth conditions, the antitoxin neutralizes the toxin's activity. However, during conditions of stress or starvation, the antitoxin is inactivated, freeing the toxin to inhibit growth and resulting in dormancy. One mechanism of growth inhibition used by several TA systems results from targeting transfer RNAs (tRNAs), either through preventing aminoacylation, acetylating the primary amino group, or endonucleolytic cleavage. All of these mechanisms inhibit translation and result in growth arrest. Many of these toxins only act on a specific tRNA or a specific subset of tRNAs; however, more work is necessary to understand the specificity determinants of these toxins. For the toxins whose specificity has been characterized, both sequence and structural components of the tRNA appear important for recognition by the toxin. Questions also remain regarding the mechanisms used by dormant bacteria to resume growth after toxin induction. Rescue of stalled ribosomes by transfer-messenger RNAs, removal of acetylated amino groups from tRNAs, or ligation of cleaved RNA fragments have all been implicated as mechanisms for reversing toxin-induced dormancy. However, the mechanisms of resuming growth after induction of the majority of tRNA targeting toxins are not yet understood. This article is categorized under: Translation > Translation Regulation RNA in Disease and Development > RNA in Disease RNA Interactions with Proteins and Other Molecules > Protein-RNA Recognition.

The role of the hok/sok locus in bacterial response to stressful growth conditions

The hok/sok locus is renowned for its plasmid stabilization effect via post-segregational killing of plasmid-free daughter cells. However, the function(s) of the chromosome-encoded loci, which are more abundant in pathogenic strains of a broad range of enteric bacteria, are yet to be understood. Also, the frequent occurrence of this toxin/antitoxin addiction system in multi-drug resistance plasmids suggests additional roles. In this study, the effects of the hok/sok locus on the growth of bacteria in stressful growth-limiting conditions such as high temperature and antibiotic burden were investigated using hok/sok plasmids. The results showed that the hok/sok locus prolonged the lag phase of host cell cultures, thereby enabling the cells to adapt, respond to the stress and eventually thrive in these growth-limiting conditions by increasing the growth rate at exponential phase. The hok/sok locus also enhanced the survival and growth of cells in low cell density cultures irrespective of unfavourable growth conditions, and may complement existing or defective SOS mechanism. In addition to the plasmid stabilization function, these effects would enhance the ability of pathogenic bacteria to establish infections and propagate the antibiotic resistance elements carried on these plasmids, thereby contributing to the virulence of such bacteria.

Dynamic instability--a common denominator in prokaryotic and eukaryotic DNA segregation and cell division

Dynamic instability is an essential phenomenon in eukaryotic nuclear division and prokaryotic plasmid R1 segregation. Although the molecular machines used in both systems differ greatly in composition, strong similarities and requisite nuances in dynamics and segregation mechanisms are observed. This brief examination of the current literature provides a functional comparison between prokaryotic and eukaryotic dynamically unstable filaments, specifically ParM and microtubules. Additionally, this mini-review should support the notion that any dynamically unstable filament could serve as the molecular machine driving DNA segregation, but these machines possess auxiliary features to adapt to temporal and spatial disparities in either system.

Competition favours reduced cost of plasmids to host bacteria

Conjugative plasmids of Gram-negative bacteria have both vertical and horizontal modes of transmission: they are segregated to daughter cells during division, and transferred between hosts by plasmid-encoded conjugative machinery. Despite maintaining horizontal mobility, many plasmids carry fertility inhibition (fin) systems that repress their own conjugative transfer. To assess the ecological basis of self-transfer repression, we compared the invasion of bacterial populations by fin(+) and fin(-) variants of the plasmid R1 using a computational model and co-culture competitions. We observed that the fin(+) variant had a modest cost to the host (measured by reduction in growth rate), while the fin(-) variant incurred a larger cost. In simulations and empirical competitions the fin(-) plasmid invaded cultures quickly, but was subsequently displaced by the fin(+) plasmid. This indicated a competitive advantage to reducing horizontal transmission and allowing increased host replication. Computational simulations predicted that the advantage associated with reduced cost to the host would be maintained over a wide range of environmental conditions and plasmid costs. We infer that vertical transmission in concert with competitive exclusion favour decreased horizontal mobility of plasmids. Similar dynamics may exert evolutionary pressure on parasites, such as temperate bacteriophages and vertically transmitted animal viruses, to limit their rates of horizontal transfer.

Plasmid R1--replication and its control

Plasmid R1 is a low-copy-number plasmid belonging to the IncFII group. The genetics, biochemistry, molecular biology, and physiology of R1 replication and its control are summarised and discussed in the present communication. Replication of R1 starts at a unique origin, oriR1, and proceeds unidirectionally according to the Theta mode. Plasmid R1 replicates during the entire cell cycle and the R1 copies in the cell are members of a pool from which a plasmid copy at random is selected for replication. However, there is an eclipse period during which a newly replicated copy does not belong to this pool. Replication of R1 is controlled by an antisense RNA, CopA, that is unstable and formed constitutively; hence, its concentration is a measure of the concentration of the plasmid. CopA-RNA interacts with its complementary target, CopT-RNA, that is located upstream of the RepA message on the repA-mRNA. CopA-RNA post-transcriptionally inhibits translation of the repA-mRNA. CopA- and CopT-RNA interact in a bimolecular reaction which results in an inverse proportionality between the relative rate of replication (replications per plasmid copy and cell cycle) and the copy number; the number of replications per cell and cell cycle, n, is independent of the actual copy number in the individual cells, the so-called +n mode of control. Single base-pair substitutions in the copA/copT region of the plasmid genome may result in mutants that are compatible with the wild type. Loss of CopA activity results in (uncontrolled) so-called runaway replication, which is lethal to the host but useful for the production of proteins from cloned genes. Plasmid R1 also has an ancillary control system, CopB, that derepresses the synthesis of repA-mRNA in cells that happen to contain lower than normal number of copies. Plasmid R1, as other plasmids, form clusters in the cell and plasmid replication is assumed to take place in the centre of the cells; this requires traffic from the cluster to the replication factories and back to the clusters. The clusters are plasmid-specific and presumably based on sequence homology.

The large plasmids found in enterohemorrhagic and enteropathogenic Escherichia coli constitute a related series of transfer-defective Inc F-IIA replicons

Forty-six of 52 (88.5%) enterohemorrhagic Escherichia coli (EHEC) strains screened carried a "common" plasmid of about 90 kb which encoded sequences homologous to the Inc F-IIA replicon. A similarly high incidence of Inc F-IIA plasmid-containing strains was observed in other groups of diarrheagenic E. coli, but not in random environmental coliform isolates. Enteropathogenic E. coli (EPEC) contain plasmids of similar properties and share a 23-kb DNA fragment with plasmids from EHEC. The common region encodes the F-IIA replication region and sequences homologous to the transfer operon of the Inc F-II plasmid R1. Sequence homology varied between plasmids isolated from different EHEC/EPEC strains with > 80% showing homology to the regions encoding the rep and par genes. Only 5% of plasmids from EHEC strains had intact sequences homologous to the DNA between these two regions, including the oriT site. Some plasmids with an apparently intact tra operon still failed to plaque F-pilus-specific phages. This is consistent with observations that the large plasmids of EHEC and EPEC are phenotypically nonconjugative. These results suggest that the large plasmids of EHEC/EPEC constitute a family of transfer-deficient Inc F-IIA plasmids with varying degrees of deletion in tra function. The evolutionary ramifications of this finding are considered.

Composite IS1-tetracycline resistance elements in aerobactin-encoding FIme plasmids from epidemic Salmonella wien

Class B tetracycline resistance determinants have been identified in two aerobactin-encoding FIme plasmids representative of those isolated from epidemic Salmonella wien. Genetic data, restriction enzyme analysis of recombinant and mutant plasmids, and Southern blot hybridizations indicate that in both plasmids the class B determinant so far found and described only on Tn10-like transposons is part of a different genetic element. This composite insertion sequence element is about 7 kilobases long and has copies of insertion sequence IS1 at the ends.

Stability of plasmid content in Salmonella wien in late phases of the epidemic history

Prevalence, genetic characteristics, and EcoRI cleavage analysis of plasmids identified in clinical strains of Salmonella wien isolated in recent years showed that the plasmid content in this serotype has remained uniform and stable over more than a decade and also late in the epidemic history. No correlation between decrease in S. wien isolations and naturally occurring systematic changes in the DNA of its most common FIme plasmid was structurally detectable.

An improved positive selection plasmid vector constructed by oligonucleotide mediated mutagenesis

An Escherichia coli plasmid vector, pUN121, has been constructed which allows for positive selection of transformants harboring DNA inserts. The positive selection of transformants harboring DNA inserts. The vector is based on plasmid pTR262 (Roberts et al. Gene, 12, (1980), 123-127) in which the tetracycline resistance gene is under transcriptional control of the repressor protein coded by the phage lambda cI gene. This plasmid has been rearranged, using in vitro recombinant techniques including oligonucleotide mediated mutagenesis to yield a smaller plasmid (4.4 kb) with unique cloning sites for EcoRI, XmaI and SmaI in addition to the unique HindIII and BclI sites. The plasmid has a functional ampicillin resistance gene and the new restriction sites (EcoRI, XmaI and SmaI) when used for cloning, give rise to tetracycline resistant transformants.

Characterization of Tn2411 and Tn2410, two transposons derived from R-plasmid R1767 and related to Tn2603 and Tn21

Two transposable elements, Tn2410 and Tn2411, were isolated from Salmonella typhimurium R-factor R1767. They have sizes of 18.5 and 18.0 kilobases, respectively. Tn2411 mediates resistance to streptomycin, sulfonamides, and mercury. In Tn2410, the streptomycin resistance gene was replaced by a gene coding for the production of the beta-lactamase OXA-2, which is responsible for ampicillin resistance. Physical and functional maps of both transposons were compared with those of Tn21, Tn4, and Tn2603. From these data it appeared that Tn21 could be an ancestral transposon from which Tn2411, Tn2410, Tn2603, and Tn4 were evolved by the addition or deletion of small DNA segments.

The instability of plasmid pON5300 after its transformation into Escherichia coli K 12

The plasmid R1drd-19 Km- and its derivative pON5300 were split by the PstI restriction enzyme; they differed in a 2.4 Mg/mol fragment which was present only in the pON5300. Both plasmids were present in 1-2 copies per chromosomal equivalent in E. coli JC5455 cells. The pON5300 is stable in its original mutagenized host but it segregates the enlarged region of the plasmid molecule after transformation into non-mutagenized E. coli JC5455. The possible explanation of this instability is discussed.

Stability of plasmids R1-19 and R100 in hyper-recombinant Escherichia coli strains and in Salmonella typhimurium strains

Plasmids R1-19 and R100 dissociate in hyper-recominant Escherichia coli strains in a way that is similar to but slower than dissociation in Salmonella typhimurium. The results presented suggest that the molecular mechanism for plasmid dissociation in hyper-recombinant E. coli strains is different than that in S. typhimurium strains.

New runaway-replication-plasmid cloning vectors and suppression of runaway replication by novobiocin

Two new cloning vectors (pBEU28 and pBEU50) with temperature-controlled runaway-replication properties are described. pBEU28 is similar to aphA+ (KanR) plasmid pBEU2 but lacks a 1.8-kb duplication which is responsible for plasmid instability. pBEU50 is an analog of pBR313 and pBR322 in that it carries bla+(AmpR), which can be used for selection, and tet+(TetR) which can be inactivated by cloning at HindIII and BamHI restriction sites. Sublethal concentrations of novobiocin were exploited to suppress runaway replication and to restore the viability of the plasmid carriers. By this method copB deletion mutants of two temperature-controlled, conditional runaway-replication plasmids were detected and isolated. The unconditional runaway-replication property of these plasmids leads us to hypothesize that there are at least two controls of plasmid R1 copy number and that the copB-dependent control is temperature-sensitive in the conditional runaway replication mutants. The novobiocin suppression of the runaway replication permitted us to clone dnaN+ on pBEU28 and to identify its presence at 42 degrees C with a dnaN59 transformation recipient which was temperature-sensitive due to a defect in the dnaN gene.

Plasmid-borne sulfonamide resistance determinants studied by restriction enzyme analysis

The relationship between sulfonamide resistance genes carried on different plasmids was investigated by restriction enzyme analysis and DNA-DNA hybridization. The results showed that sulfonamide resistance mediated by different plasmids is determined by the production of at least two different types of drug-resistant dihydropteroate synthase. Plasmids pGS01, pGS02, and R22259, found in bacteria isolated from patients in Swedish hospitals, contained identical sulfonamide resistance genes, which were also identical to those of plasmids R1, R100, R6, and R388. These latter plasmids, which have been well studied in different laboratories, were originally from clinical isolates from different parts of the world. Two other clinically isolated plasmids, pGS04 and pGS05, were shown to contain sulfonamide resistance determinants of a completely different type.

Isolation of the kanamycin resistance region (Tn2350) of plasmid R1drd-19 as an autonomous replicon

We have isolated a circular form of Tn2350, an IS1-flanked kanamycin resistance transposon forming part of the plasmid R1drd-19. This circle (pTn2350::9.6 kilobases) contains a single IS1 element and probably arises by recombination between the two directly repeated Is1 sequences of Tn2350. It can be used to transform Escherichia coli to kanamycin resistance. It is capable of autonomous replication but is not maintained stably in dividing cells and segregates under nonselective conditions. Cloning of a segment of pTn2350 on a conditional plasmid vector allowed us to assign the replication functions of this plasmid to a 1.6-kilobase restriction fragment. The plasmid R1drd-19 can thus be considered as a cointegrate between two replicons separated by IS1 sequences.

Cloning vectors derived from bacterial plasmids

A wide variety of plasmid cloning vectors, most of which utilize the basic Co1E1I replicon have been constructed. Utilizing these vectors, in conjunction with the newly developed techniques of gene isolation and oligonucleotide synthesis, essentially any gene which can be identified can be cloned. We anticipate that future work in this area will be directed at improving techniques for the regulated expression of cloned genes and the further development of plasmid replicons in which the copy number can be readily controlled.

Physical characterization of the plasmid pON5300

The antibiotic resistance plasmid R1drd19Km- which has spontaneously lost its kanamycin resistance marker, and its derivative pON5300, were analysed using the restriction endonucleases SalI, BamHI, HindIII and EcoRI. The fragment patterns were compared with that of the R1drd19 and the fragments responsible for kanamycin resistance were found to be missing in R1drd19Km- and pON5300. In these plasmids a 7 Mg/mol EcoRI fragment was observed instead of the D (6.3 Mg/mol) fragment of R1drd19. Further a new 6 Mg/mol EcoRI restriction fragment was observed in pON5300. Using double digestions it was shown that the new fragment does not carry restriction sites for HindIII, BamHI and SalI endonucleases. The non-homology of the analysed plasmid was proved electron microscopically by heteroduplex techniques. The possibility of amplification in the regulatory region for the expression of R-determinants in pON5300 is discussed.

The mechanism of resistance to streptomycin in Escherichia coli. Functional analysis of the permeability barrier of cells harbouring the R1 drd-19Km- plasmid

The mechanism of phenotypically altered SM resistance in mutants of Escherichia coli JC5455 (Rldrd-19Km-) lrs and JC5455 (pON5300) was compared with that of the standard strain JC5455 (Rldrd-19Km-). On analyzing the membrane polypeptides in polyacrylamide gel both mutants were found to possess a protein spectrum different from that of ths standard strain. Transport of D-xylose and L-arginine was the same in all strains, transport of L-proline was decreased in JC5455 (pON5300) which may indicate a mutational interference with energy metabolism. The basic uptake of dihydrostreptomycin was the same in all strains but there were differences after preincubation of cells with streptomycin or glucose. The increased resistance of JC5455 (Rldrd-19Km-) lrs may be due to observed quantitative differences in membrane polypeptides that might play a role in the binding and functional expression of aminoglycoside-3'adenylyl transferase which modifies streptomycin. The increased sensitivity toward streptomycin in JC5455 (pON5300) can be explained by a mutation due to N-methyl-N'-nitro-N-nitrosoguanidine in the host cell since this change of sensitivity to streptomycin could not be transferred by transformation into a nonmutagenized strain. The coincidence of inducibility of increased transport of streptomycin by this antibiotic and the altered frequency of reversion to high levels of streptomycin resistance in JC5455 (pON5300) and in the transformant JC5455 (pON5302) may indicate that the altered reversibility toward phenotypically high resistance to streptomycin is a property of pON5300 and is transferred by transformation.

Physical and genetic organization of the IncN-group plasmid pCU1

A restriction endonuclease-cleavage map of the IncN group plasmid pCU1 was constructed. Deletion mutants of the plasmid were obtained by in vivo or in vitro methods. Comparison of the restriction maps of these mutants to that of pCU1 enables one to assign the known functions of the plasmid to particular regions on the plasmid DNA. For different enzymes, the number and distribution of restriction sites on pCU1 is compared to that of other IncN and related plasmids.

A mutant defective in partitioning of composite plasmid Rms201

Escherichia coli harboring mutant plasmids defective in maintenance stability (from the conjugative plasmid Rms201) showed a wide distribution of ampicillin resistance levels, as well as increased frequency of plasmid loss from the cell. The amounts of covalently closed circular deoxyribonucleic acid of mutant plasmid Rms268 and parental plasmid Rms201 per chromosome were 5.3 and 6.1%, respectively. The beta-lactamase activities of strains W3630(Rms268) and W3630(Rms201) were 0.56 and 0.44 U/mg of protein, respectively. Frequency of plasmid loss from W3630(Rms268) was about 0.8 to 1.2% per cell generation, 100 times more than that of the wild-type strain. Ampicillin resistance levels of the colonies harboring the mutant plasmid showed a wide distribution, from low (100 micrograms/ml) to high (1,600 micrograms/ml). A miniplasmid (pMS268) with a mass of 7 X 10(6) daltons and encoding ampicillin resistance was isolated from Rms268. Frequency of pMS268 loss from W3630(pMS268) was about 0.8 to 1.9% per cell generation. W3630(pMS268) also showed a wide range of distribution in the levels of ampicillin resistance. These results indicated that the copies of Rms268 in E. coli did not segregate evenly between daughter cells at cell division and that the gene involved was located on the miniplasmid.

Isolation and characterization of lambda phages carrying the basic replicon of the resistance plasmid R1

Specialized transducing lambda phages, lambda oriR1, harboring DNA from the resistance plasmid R1drd-19 and its copy mutant pKN103 were isolated. From measurements of CCC-DNA content it is concluded that upon infection the phages can establish themselves as self-replicating plasmids in recA hosts lysogenic for lambda. It is thought that this bypassing of lambda immunity is due to the presence of the R1 origin of replication. The plasmids are sensitive to the incompatibility expressed by plasmid R1. This has been shown mainly by transduction of lambda oriR1 into recipients containing R1 plasmids or plasmid pBR322 carrying the basic replicon. We were able to demonstrate that a copy mutant of plasmid R1 was insensitive to copA+, but sensitive to the concerted action of PstI fragments F1 and F2. This mutant was previously assumed to be of dominant type. Physical mapping of the lambda oriR1 derivatives verified that they carry the basic replicon of plasmid R1. The plasmids are not stably maintained, but are lost in a frequency of 1%-2% per cell generation, which is consistent with their lack of the R1par region.

Copy number control and incompatibility of plasmid R1: identification of a protein that seems to be involved in both processes

Investigations into the genetic determinants for incompatibility of miniplasmids and hybrid replicons constructed from wide type and mutant R1 revealed the presence of an incompatibility function at the junction f two small PstI fragments. These two fragments were not distinguished in earlier experiments since they have the same mobility on agarose gels. This incompatibility function is distinct from other inc-determinants of R1 (Kollek and Goebel 1979; Molin and Nordström, 1980) and independent of R1-type replication. By means of specific deletions and subcloning of DNA fragments, the location of this new inc-determinant could be determined further. After deletion of this inc-determinant from inc-determinant from miniplasmids, a 5-fold increase in copy number was observed which could then be reduced to a copy number of about 1 plasmid per cell by complementation with hybrid plasmids having this function. Incompatibility of miniplasmids deleted in this determinant is not reduced, whereas analogous deletions introduced into recombinant plasmids nearly abolished their incompatibility. This determinant seems to exert strong incompatibility only when cloned on pBR322. Therefore, its main function is plasmid R1 is probably restricted to copy control. The appearance of low copy numbers of of miniplasmids carrying this determinant and of trans-acting copy control and strong incompatibility exerted by hybrid plasmids is consistently correlated with the presence of a protein of 11,000 molecular weight, synthesized in relatively large amounts in Escherichia coli minicells.

Physical and functional mapping of Tn2603, a transposon encoding ampicillin, streptomycin, sulfonamide, and mercury resistance

A map of cleavage sites for restriction endonuclease EcoRI, BamHI, HindIII, and SalI on Tn2603, a transposon encoding resistance to ampicillin, streptomycin, sulfonamide, and mercury, was constructed by an analysis of restriction cleavage patterns of plasmid pMK1.::Tn2603 and its deletion derivative. By cloning the fragments generated from pMK1.::Tn2603 with these restriction endonucleases to a pACYC184 plasmid vehicle, the regions necessary for expression of resistance were located on the restriction cleavage map of Tn2603. Ampicillin, streptomycin, and sulfonamide-resistance genes were mapped in a cluster on the region between the center and the right and the mercury-resistance gene was located to the left of the map. The final functional map of Tn2603 was compared with those of Tn4 and Tn21 and the evolutional relationships between them were discussed.

The structure of R1drd19: a revised physical map of the plasmid

We have analyzed derivatives of the plasmid R1drd19 carrying the transposon Tn10 by electron microscopy following denaturation and renaturation of the molecules, and by digestion with various restriction enzymes, gel electrophoresis and Southern blotting. We show: 1) that the published restriction map of R1drd19 is inconsistent with our results. We present a modified map which is consistent with our data. 2) that R1drd19 carries a single resident copy of the element IS10 which is normally associated with Tn10 as an inverted repeat, and 3) that R1drd19 carries three copies of the insertion element IS1 in the resistance determinant region.

Isolation and characterization of new copy mutants of plasmid R1, and identification of a polypeptide involved in copy number control

Site-specific deletions and insertions in the replication region of plasmid R1 have generated a new class of copy mutants that are present in the cell with 10-15-fold increased copy number. All mutations described inactivate a copy number control gene which is distinct from another cop inc gene that was identified previously (Molin and Nordström 1980). Insertion of the lac operon lacking the normal lac promoter has been used to determine the direction of transcription of this cop gene. The mutants may all be complemented by wild-type plasmid derivatives and are thus recessive. In incompatibility tests with wild-type R1 plasmids, these mutants are indistinguishable from the wild-type plasmid. It therefore seems that this cop function does not play an important role for the incompatibility function. A polypeptide, molecular weight 11,000, has been identified as being the product of this cop gene.

Determination of the functions of hemolytic plasmid pHly152 of Escherichia coli

The alpha-hemolytic Escherichia coli strain PM152 harbors three transmissible plasmids, which have molecular weights of 65 X 10(6) (pA152), 41 X 10(6) pHly152), and 32 X 10(6) (pC152). Plasmids pHly152 and pC152 belong to incompatibility groups J2 and N, respectively. By transforming E. coli K-12 with isolated plasmids, we showed that the genetic determinant required for hemolysis was located entirely on plasmid pHly152, and a physical map of this plasmid was constructed. By transposon mutagenesis, a deoxyribonucleic acid segment of about 3.5 X 10(6) daltons was identified as being essential for hemolysis. Most of the EcoRI and HindIII fragments of the hemolytic plasmid pHly152 were cloned by using pACYC184 and RSF2124 as vectors. Two classes of Tn3-induced hemolysis-negative mutants could be complemented by recombinant plasmids carrying fragments from the hemolysis region of pHly152, whereas a third class could be restored to hemolytic activity only by recombination between the mutant plasmids and a suitable recombinant deoxyribonucleic acid. These data suggest that there are at least three clustered cistrons which are required for hemolysis. Other EcoRI and HindIII fragments of pHly152 were identified as being essential for replication, incompatibility, transfer, and restriction.

Plasmid replication functions: two distinct segments of plasmid R1, RepA and RepD, express incompatibility and are capable of autonomous replication

The genetic determinants for replication and incompatibility of plasmid R1 were investigated by gene cloning methods, and three types of R1 miniplasmid derivatives were generated. The first, exemplified by plasmid pKT300, consisted of a single BglII endonuclease-generated deoxyribonucleic acid fragment derived from the R1 region that is located between the determinants for conjugal transfer and antibiotic resistance. Two types of miniplasmids could be formed from PstI endonuclease-generated fragments of pKT300. One of these, which is equivalent to miniplasmids previously generated from plasmids R1-19 and R1-19B2, consisted of two adjacent PstI fragments that encode the RepA replication system of plasmid R1. The other type contained a segment of R1, designated the RepD replication region, that is adjacent to the RepA region and that has not been identified previously as having the capacity for autonomous replication. Plasmid R1, therefore, contained two distinct deoxyribonucleic acid segments capable of autonomous replication. The RepA-RepD miniplasmid pKT300 had a copy number about eightfold higher than that of R1 and hence lacked a determinant for the regulation of plasmid copy number. Like R1, it was maintained stably in dividing bacteria. RepA miniplasmids had copy numbers which were two- to fourfold higher than that of R1 (i.e., which were lower than that of pKT300) and were maintained slightly less stably than those of pKT300 and R1. The RepD miniplasmid was not maintained stably in dividing bacteria. Previous experiments have shown that incompatibility of IncFII group plasmids is specified by a plasmid copy control gene. Despite the fact that RepA miniplasmids of R1 were defective in copy control, they nevertheless expressed incompatibility. This suggests that two genes are responsible for plasmid copy control, one that specifies incompatibility and is located on RepA miniplasmids and another that is located outside of, but adjacent to, the RepA replication region. Hybrid plasmids composed of pBR322 and one PstI fragment from the RepA region, P-8, exhibited incompatibility towards R2 and RepA miniplasmids but not the RepD miniplasmid, whereas hybrids composed of pBR322 and the PstI fragment of the RepD region, P-3, exhibited incompatibility towards R1 and the RepD miniplasmid but not RepA miniplasmids. These results indicate that the two replication systems are functionally distinct and that, although the RepA system is the principal replication system of R1, the RepD system also plays a role in the maintenance of this plasmid.

Isolation of an IS1 flanked kanamycin resistance transposon from R1drd19

We have isolated and identified an IS1-flanked transposon from the plasmid R1drd19. This transposon specifies resistance to kanamycin and is 10.4 kg long. It exhibits a frequency of transposition two orders of magnitude lower than that of the smaller, IS1-flanked transposon Tn9. We have named it Tn2350.

Partitioning of plasmid R1 in Escherichia coli. I. Kinetics of loss of plasmid derivatives deleted of the par region

The stability of inheritance of plasmid R1drd-19 was tested. The copy number of the plasmid was determined in two different ways: As the ratio between covalently closed circular DNA and chromosomal DNA, and by quantitative determination of single-cell resistance to ampicillin. In the latter case, strains carrying the R1 ampicillin transposon Tn3 on prophage lambda was used as standard. The values were transformed to copy number per cell by using the Cooper-Helmstetter model for chromosome replication as well as by determination of chromosomal DNA per cell by the diphenylamine method. The copy number was found to be five to six per cell (or about four per newborn cell). Nevertheless, plasmid R1drd-19 was found to be completely stably inherited. This stability was shown not to be due to retransfer of the plasmid by the R1 conjugation system, since transfer-negative derivatives of the plasmid were also completely stably inherited. Smaller derivatives of plasmid R1drd-19 were found to be lost at a frequency of about 1.5% per cell generation. The copy-number control was not affected in these miniplasmids, since their copy numbers were the same as that of the full size plasmid. Quantitatively, the instability of the miniplasmids was in accord with random partitioning. It is, therefore, suggested that the plasmid R1drd-19 carries genetic information for partitioning (par) of plasmid copies at cell division, and that the par mechanism is distinct from the copy number control (cop) system. Finally, the par gene maps on the resistance transfer part of the plasmid, but far away from the origin of replication and the so-called basic replicon; this is in accord with the approximate location of the repB gene (Yoshikawa, 1974, J. Bacteriol., 118, 1123-1131).

Expression of Ti-plasmid DNA in E. coli: comparison of homologous fragments cloned from Ti plasmids of Agrobacterium strains C58 and Ach5

Fragments of two Ti plasmids from Agrobacterium tumefaciens were cloned and studied for cross-hybridization. Homologous fragments were further investigated for areas of strong homology and mapped with various restriction enzymes. The fragments were inserted in two directions and the recombinant plasmid was brought into E. coli strains producing minicells. About five proteins were found to be coded by those fragments of the Ti plasmids.

Transposon-mediated conjugational transmission of nonconjugative plasmids

When coresident with conjugative plasmid pNC21, the nonconjugative deletion F-prime pJC59, which retains the F transfer origin oriT, was transmitted to transconjugants at a frequency comparable to that of pNC21. In addition, pJC59 was transmitted as an independent plasmid, physically separate from pNC21, an example of plasmid donation. In contrast, two plasmids that are derived from F and deleted for the oriT site, pJC61 and pML31, were transmitted at frequencies 10(4) lower than that of pNC21. This low-frequency transmission was associated with the appearance of a new plasmid in the transconjugants. In the case of pML31, we determined that this new plasmid was a recombinant composed of pNC21 and pML31, the latter flanked by two copies of transposable element Tn3. We believe that this recombinant plasmid was formed as an intermediate in the transposition of Tn3 from pNC21 to pML31 and was the vehicle for conjugational transmission of pML31 genes by a process known as plasmid conduction.

Integration of specialized transducing bacteriophage lambda cI857 St68 h80 dgnd his by an unusual pathway promotes formation of deletions and generates a new translocatable element

Molecular and genetic studies have revealed that several illegitimate recombinational events are associated with integration of the specialized transducing bacteriophage lambda cI57 St68 h80 dgnd his into either the Escherichia coli chromosome or into a plasmid. Most Gnd+ His+ transductants did not carry the prophage at att phi-80, and 10% were not immune to lambda, i.e., "nonlysogenic." Integration of the phage was independent of the phage Int and Red gene products and of the host's general recombination (Rec) system. In further studies, bacterial strains were selected which carried the phage integrated into an R-factor, pSC50. Restriction endonuclease analysis of plasmid deoxyribonucleic acid (DNA) purified from these strains showed that formation of the hybrid plasmids resulted from recombination between a single region of pSC50 and one of several sites within the lambda-phi 80 portion of the phage. Furthermore the his-gnd region of the phage, present in the chromosome of one nonlysogenic transductant, was shown to be able to translocate to pSC50. Concomitant deletion of phage DNA sequences or pSC50 DNA was frequently observed in conjunction with these integration or translocation events. In supplemental studies, a 22- to 24-megadalton segment of the his-gnd region of the chromosome of a prototrophic recA E. coli strain was shown to translocate to pSC50. One terminus of this translocatable segment was near gnd and was the same as a terminus of the his-gnd segment of the phage which translocated from the chromosome of the nonlysogenic transductant. These data suggest that integration of lambda cI857 St 68 h80 dgnd his may be directed by a recombinationally active sequence on another replicon and that the resulting cointegrate structure is subject to the formation of deletions which extend from the recombinationally active sequence. Translocation of the his-gnd portion of the phage probably requires prior replicon fusion, whereas the his-gnd region of the normal E. coli chromosome may comprise a discrete, transposable element.

Control of plasmid R1 replication: functions involved in replication, copy number control, incompatibility, and switch-off of replication

A small derivative of plasmid R1 was used to integratively suppress a chromosomal dnaA(Ts) mutation. The strain obtained grew normally at 42 degrees C. The integratively suppressed strain was used as recipient for various plasmid R1 derivatives. Plasmid R1 and miniplasmid derivatives of R1 could be established in the strain that carried an integrated R1 replicon, but they were rapidly lost during growth. However, plasmids also carrying ColE1 replication functions were almost completely stably inherited. The integratively suppressed strain therefore allows the establishment of bacteria diploid with respect to plasmid R1 and forms a useful and sensitive system for studies of interaction between plasmid R1 replication functions. Several of the chimeric plasmids caused inhibition of growth at high temperatures. All plasmids that inhibited growth carried one particular PstI fragment from plasmid R1 (the PstI F fragment), and in all cases the growth inhibition could be ascribed to repression of initiation of chromosome replication at 42 degrees C, i.e., they carry a trans-acting switch-off function. Furthermore, the analogous PstI fragments from different copy mutants of plasmid R1 were analyzed similarly, and one mutant was found to lack the switch-off function. The different chimeric plasmids were also tested for their incompatibility properties. All plasmids that carried the switch-off function (and no other plasmids) also carried R1 incompatibility gene(s). Since the PstI F fragment, which is present on all these plasmids, is very small (0.35 x 10(6)), it is suggested that the switch-off regulation of replication (by an inhibitor), incompatibility, and copy number control are governed by the same gene.

A transposon, Tn732, encoding gentamicin/tobramycin resistance

Gentamicin and tobramycin are important antibiotics in the treatment of hospital infections because of their activity against a wide range of bacterial genera. With their increasing use, bacteria resistant to these drugs have appeared, the resistance being frequently plasmid determined. The resistance genes determine various enzymes that modify and inactivate the drugs and there is association between particular gentamicin/tobramycin resistance genes and plasmids of particular groups, implying that acquisition of such a gene by any plasmid is a rare event. We now report the identification of a transposon or 'jumping gene' encoding the gentamicin/tobramycin adenylylating enzyme, ANT(2"), on a plasmid of incompatiblity group FII (IncFII).

Plasmid cistrons controlling synthesis and excretion of the exotoxin alpha-haemolysin of Escherichia coli

The synthesis and secretion of the toxic exoprotein alpha-haemolysin of E. coli PM152 is coded by the transmissible plasmid pHly152 (41 x 10(6) dalton) as shown by the transformation of the plasmid DNA and the isolation of mutants that are specifically altered in the synthesis and transport of haemolysin. These mutants were obtained by chemical mutagenesis and insertion of the ampicillin transposon (Tn3) into pHly152. Tn3 transposition was also used for the identification and the location of the cistrons on pHly152 essential for haemolysis. The EcoRI and HindIII fragments of the haemolytic plasmid pHly152 were cloned and used for the complementation of the haemolysis negative Tn3 insertion mutants. A DNA segment of 3.2 x 10(6) dalton could be thus identified which consists of at least three clustered cistrons necessary for haemolysis. Two of these cistrons are required for the formation of active haemolysin. At least one other cistron seems to be involved in the secretion of active haemolysin through the outer membrane of E. coli. The gene products determined by these cistrons were identified in minicells of E. coli. Their molecular properties were determined and their possible function in the formation and secretion of haemolysin will be discussed.

Plasmids with temperature-dependent copy number for amplification of cloned genes and their products

Miniplasmids (pKN402 and pKN410) were isolated from runaway-replication mutants of plasmid R1. At 30 degrees C these miniplasmids are present in 20--50 copies per cell of Escherichia coli, whereas at temperatures above 35 degrees C the plasmids replicate without copy number control during 2--3 h. At the end of this period plasmid DNA amounts to about 75% of the total DNA. During the gene amplification, growth and protein synthesis continue at normal rate leading to a drastic amplification of plasmid gene products. Plasmids pKN402 (4.6 Md) and pKN410 (10 Md) have single restriction sites for restriction endonucleases EcoRI and HindIII; in addition plamid pKN410 has a single BamHI site and carries ampicillin resistance. The plasmids can therefore be used as cloning vectors. Several genes were cloned into these vectors using the EcoRI sites; chromosomal as well as plasmid-coded beta-lactamase was found to be amplified up to 400-fold after thermal induction of the runaway replication. Vectors of this temperature-dependent class will be useful in the production of large quantities of genes and gene products. These plasmids have lost their mobilization capacity. Runaway replication is lethal to the host bacteria in rich media. These two properties contribute to the safe use of the plasmids as cloning vehicles.

Clustering of genes involved in replication, copy number control, incompatibility, and stable maintenance of the resistance plasmid R1drd-19

Plasmid R1drd-19 is present in a small number of copies per cell of Escherichia coli. The plasmid was reduced in size by in vivo as well as in vitro (cloning) techniques, resulting in a series of plasmid derivatives of different molecular weight. All plasmids isolated contain a small region (about 2 x 10(6) daltons of deoxyribonucleic acid) of the resistance transfer factor part of the plasmid located close to one of the IS1 sequences that separates the resistance transfer factor part from the resistance determinant. All these derivatives were present at the same copy number, retained the incompatibility properties of plasmid R1drd-19, and were stably maintained during cell division. Genes mutated to yield copy mutations also were found to be located in the same region.