Physical properties and mechanism of transfer of R factors in Escherichia coli

The activity of a single-stranded promoter of plasmid ColIb-P9 depends on its secondary structure

The leading region of the conjugal bacterial plasmid ColIb-P9 contains three dispersed repeats of a 328 bp sequence homologous to Frpo, a sequence from plasmid F that acts as a promoter in single-stranded DNA. One of these sequences, ssi3, inactive in the double-stranded form, promoted in vitro transcription exclusively from the single strand that is transferred during conjugation. Promoter activity was dependent on the presence of RNA polymerase holoenzyme containing sigma 70. Transcription initiated from the position predicted from folding the single-stranded DNA to form a pseudo double-stranded hairpin structure containing recognizable -35 and -10 promoter elements. Footprinting of RNA polymerase holoenzyme on single-stranded ssi3 DNA was consistent with this suggestion. Mutagenesis of the putative -35 region inactivated the promoter, but random mutations in the -10 region had little effect. The putative -10 region is a poor match to the consensus sequence and contains mismatched bases. Elimination of these mismatches invariably destroyed single-strand promoter activity. These observations reveal the crucial contribution of the unpaired bases in the -10 region in potentiating the formation of the productive open complex with RNA polymerase.

Selection of plasmid molecules for conjugative transfer and replacement strand synthesis in the donor

Plasmid selection and strand replacement synthesis in donor cells during conjugative transfer was examined by a procedure involving electroporation of test plasmid DNA, containing a base pair mismatch, into donor cells prior to mating. Multiple copies of the plasmid were transferred from a donor cell that allowed vegetative replication of the plasmid. Under conditions non-permissive for vegetative replication, there were further rounds of transfer after a lag period. Strand replacement in the donor did not depend solely on the initiation mechanism for vegetative replication, indicating a conjugation-specific mechanism was also available. The lag period between first and second rounds of transfer argues against the transfer of multiple copies into recipients by the spooling of copies generated on a master molecule by rolling-circle replication.

Expression of leading region genes on IncI1 plasmid ColIb-P9: genetic evidence for single-stranded DNA transcription

The leading region of a plasmid is the first sector to enter the recipient cell in bacterial conjugation. This sector of IncI1 plasmid ColIb-P9 includes genes that are transcribed in a transient pulse early in the conjugatively infected cell to promote establishment of the immigrant plasmid. Evidence is presented that the burst of gene expression is regulated by a process which is independent of a repressor but dependent on the orientation of the genes on the unique plasmid strand transferred in conjugation. The nucleotide sequence of 11.7 kb of the leading region was determined and found to contain 10 ORFs; all are orientated such that the template strand for transcription corresponds to the transferred strand. The leading region contains three dispersed repeats of a sequence homologous to a novel promoter in ssDNA described by H. Masai & K. Arai (1997, Cell 89, 897-907). It is proposed that the repeats are promoters that form in the transferring strand of ColIb to support transient transcription of genes transferred early in conjugation.

Unidirectional transfer of broad host-range plasmid R1162 during conjugative mobilization. Evidence for genetically distinct events at oriT

A segment of R1162 DNA containing genes for conjugative mobilization (Mob) and the origin of transfer (oriT) was integrated into the Escherichia coli chromosome. Bacterial genes were transferred in a polar fashion during conjugative mobilization of the integrated segment by a self-transmissible plasmid vector. The direction of transfer, together with the properties of mutated derivatives of oriT, indicate that initial cleavage of oriT, and subsequent religation after transfer, entail different mechanisms that can be distinguished genetically.

Transfer of tra proteins into the recipient cell during bacterial conjugation mediated by plasmid ColIb-P9

Selective transfer of the two products of the ColIb primase gene, sog, from donor to recipient cell during conjugation was demonstrated by two independent methods. The transfer of these tra proteins was unidirectional and dependent on DNA transfer. The Sog polypeptides were localized to the cytoplasm of the donor cell, but they appeared to interact with other tra gene products located in the inner membrane. After cell mating, the transferred polypeptides were found to be in the cytoplasm of the recipient cell, and it is estimated that as many as 500 Sog polypeptides were transferred per round of conjugation. It is proposed that these proteins are transferred as a result of an interaction with the single-stranded DNA and that the transferred strand may be coated with Sog polypeptides.

Direction of conjugative transfer of IncI1 plasmid ColIb-P9

The origin-of-transfer region of ColIb-P9 was inserted into a lambda prophage to give a bacterial chromosome mobilizable by the parental conjugative plasmid. The polarity of mobilization of chromosomal genes indicated that ColIb-P9 transfer is unidirectional, such that the transfer genes adjacent to oriT enter the recipient cell last.

Physical and genetic analyses of the Inc-I alpha plasmid R64

A 126-kilobase (kb) physical and genetic map of the Inc-I alpha plasmid R64 was constructed by using the restriction enzymes, BamHI, SalI, XhoI, HindIII, and EcoRI. The replication (Rep) and incompatability (Inc) functions of this plasmid were located in a 1.75-kb segment of an EcoRI fragment, E10 (3.3 kb). In addition, the genes determining growth inhibition of phage BF23 (Ibf), suppression of dnaG ( Sog ), resistance to tetracycline (Tetr), and resistance to streptomycin ( Strr ) were located on the 5.5-kb HindIII-XhoI fragment, the 8.1-kb EcoRI fragment (E5), the 4.6-kb HindIII fragment (H8), and the 4.1-kb HindIII fragment (H10), respectively. The map of R64 was compared with that of ColIb, which belongs to the Inc-I alpha group.

DNA primase of plasmid ColIb is involved in conjugal DnA synthesis in donor and recipient bacteria

The sog gene of the IncI alpha group plasmid ColIb is known to encode a DNA primase that can substitute for defective host primase in dnaG mutants of Escherichia coli during discontinuous DNA replication. The biological significance of this enzyme was investigated by using sog mutants, constructed from a derivative of ColIb by in vivo recombination of previously defined mutations in a cloned sog gene. The resultant Sog- plasmids failed to specify detectable primase activity and were unable to suppress a dnaG lesion. These mutants were maintained stably in E. coli, implying that the enzyme is not involved in vegetative replication of ColIb. However, the Sog- plasmids were partially transfer deficient in E. coli and Salmonella typhimurium matings, consistent with the hypothesis that the normal physiological role of this enzyme is in conjugation. This was confirmed by measurements of conjugal DNA synthesis. Studies of recipient cells have indicated that plasmid primase is required to initiate efficient synthesis of DNA complementary to the transferred strand, with the protein being supplied by the donor parent and probably transmitted between the mating cells. Primase specified by the dnaG gene of the recipient can substitute partially for the mutant enzyme, thus providing an explanation for the partial transfer proficiency of the mutant plasmids. Conjugal DNA synthesis in dnaB donor cells was deficient in the absence of plasmid primase, implying that the enzyme also initiates synthesis of DNA to replace the transferred material.

Evidence for two genetically distinct DNA primase activities specified by plasmids of the B and I incompatibility groups

Plasmid ColIb-P9 of the I alpha incompatibility group is known to encode a DNA primase that acts in the conjugal transfer of the plasmid and can substitute for mutant dnaG gene product in vegetative replication of the Escherichia coli chromosome. The relevant genetic determinant (sog) has previously been cloned into a small multicopy vector plasmid. Prototype IncB plasmid R16 also suppresses host dnaG mutations. The equivalent gene(s) (pri) of R16 was cloned into plasmid pBR325 and shown by filter hybridization studies to be different from the ColI primase gene(s). The cloned fragment carrying the pri determinant encoded an enzyme which could initiate DNA synthesis in vitro on single-stranded phage M13 DNA template, but which was antigenically distinct from ColI primase. We used the cloned primase genes as probes in colony hybridization screening of strains carrying plasmids of the IncI complex and IncB group. Plasmids R64, R144 (I alpha), R621a (I gamma), RIP72, and R864a (B) contained nucleotide sequences homologous with the cloned ColI sog genes. Plasmids R805 (I xi), R724, (I delta), TP125, and PLG101 (B) showed sequence homology with the R16 pri determinant.

Plasmid RP4 specifies a deoxyribonucleic acid primase involved in its conjugal transfer and maintenance

We surveyed plasmids representative of most incompatibility groups for their conferred deoxyribonucleic acid (DNA) primase activity. RP4 (IncP) was one of the few with such activity although, unlike the derepressed IncIalpha plasmids (which also specify a primase), it did not suppress the dnaG mutation. Using deletion and Tn7 derivatives of RP4, we located the presumed primase structural gene (pri) in the 37- to 42-kilobase region. Tn7 insertions in the adjacent Tra1 region also reduced or caused overproduction of primase. We purified the RP4 primase to a single polypeptide of molecular weight 118,000. It is an anisometric molecule and functions as a monomer, initiating complementary strand synthesis on phi X174 DNA in Escherichia coli dnaG cell extracts in the presence of ribonucleotide triphosphates and rifampin. It is immunologically unrelated to either the E. coli dnaG or the IncIalpha plasmid-specified DNA primases. RP4 pri mutants conjugated with a lower efficiency into some bacterial species, including Salmonella typhimurium. Back-transfer experiments showed that this effect was recipient specific. There was also a comparable reduction in mobilization efficiency of R300B by RP4 pri into such recipients. Loss of RP4 primase led to detectable plasmid instability. The RP4-specified primase therefore seems to serve two functions: the single DNA strand transferred during conjugation is primed by it in the recipient cell, and it appears to be necessary for the efficient priming of discontinuous plasmid DNA replication despite the presence of the chromosomal priming system.

Role of ribonucleic acid synthesis in conjugational transfer of chromosomal and plasmid deoxyribonucleic acids

A strain of Escherichia coli K-12 containing mutations that allow for the experimental control of RNA and DNA syntheses was constructed to investigate the role that RNA synthesis plays in conjugational DNA transfer when DNA replication is inhibited. The mutations possessed by this strain and its donor derivatives include: (i) thyA, which blocks synthesis of dTMP, causing a requirement for thymine; (ii) deoC, which blocks breakdown of deoxyribose 5-phosphate, permitting growth with low levels of thymine; (iii) pyrF, which blocks synthesis of UMP from OMP, imposing a requirement for uridine; (iv) cdd and pyrG, which block the deamination of cytidine to uridine and the synthesis of CTP from UTP, respectively, causing a requirement for cytidine; (v) codA and codB, which block the deamination of cytosine to uracil and cytosine transport, respectively, preventing the substitution of cytosine for cytidine; and (vi) dnaB, which blocks vegetative but not conjugational DNA replication at 42 degrees C. DNA synthesis can be blocked in the donor strains by the addition of excess uridine when exogenous thymine is not present. We found that RNA synthesis can also be blocked by addition of excess uridine when exogenous cytidine is not present. Blocking RNA synthesis prior to mating, under conditions in which DNA synthesis either is or is not inhibited, depresses DNA transfer. However, under conditions in which DNA synthesis is inhibited, the blocking of RNA synthesis immediately after mating has commenced had no effect on continued conjugational transfer of DNA. Thus, RNA synthesis is needed to initiate but not to continue conjugational DNA transfer.

A novel priming system for conjugal synthesis of an IncI alpha plasmid in recipients

Synthesis of DNA complementary to the transferred strand of an IncI alpha plasmid has been shown previously to require DNA polymerase III. The possible involvement of the two defined priming proteins of Escherichia coli K12, RNA polymerase and primase, in initiating this conjugal DNA synthesis had been examined. Primase was inactivated using temperature-sensitive dnaG3 mutants and RNA polymerase was inhibited using rifampicin. When these two proteins were simultaneously inactivated in both parental strains, the average recipient synthesised at least one single-stranded equivalent of R144drd-3 before the rifampicin-treated donors lost the ability to transmit DNA. It is proposed that the product of a plasmid transfer gene is responsible for initiating this DNA synthesis in recipients. The results imply that this protein is supplied by the donors.

A DNA primase specified by I-like plasmids

An enzyme has been isolated from Escherichia coli strains harboring the I-like plasmid R64drd11, which is capable of initiating DNA synthesis on the circular, single-stranded DNA of phages phi X174, fd, and G4. In the conversion of these templates to duplex forms in vitro, the enzyme can substitute for the functions of E. coli dna B-dnaB-dnaC-dnaG proteins, E. coli RNA polymerase, and E. coli dnaG protein, respectively. The enzyme requires all four ribonucleoside triphosphates for optimal activity, although a combination of ATP, CTP, and GTP can almost completely satisfy the rNTP requirement. The enzyme appears to cooperate specifically with DNA polymerases III because single-stranded DNA-dependent synthesis takes place in extracts deficient in DNA polymerases I and II but not in extracts from a dnaZ mutant. Highly purified enzyme preparations consist mostly of two major polypeptides, Mr 140,000 and 180,000, when analyzed by sodium dodecyl sulfate gel electrophoresis. These polypeptides cosediment with the enzyme activity through a glycerol gradient with a sedimentation coefficient of 3.6 S. DNA priming activity in extracts of E. coli strains harboring the mutant plasmids R64drd11 or ColIdrd1, which are derepressed in functions of conjugational DNA transfer, severalfold higher than the activity from strains carrying the corresponding wild-type plasmid. This correlation suggests that the enzyme may play a role in conjugational DNA synthesis.

A colI-specified product, synthesized in newly infected recipients, limits the amount of DNA transferred during conjugation of Escherichia coli K-12

The amount of ColI DNA transferred between mating cells of Escherichia coli K-12 increased about fourfold when rifampin-resistant donors were mated with sensitive recipients in the presence of the drug. Conjugational synthesis of ColI in dnaB recipients, shown primarily to reflect conversion of the transferred DNA into double-stranded material, was also enhanced when the recipients were treated with either rifampin or streptomycin. It is suggested that the amount of ColI transfer is normally limited by the synthesis of one or more proteins in the newly infected recipients. The protein is thought to be plasmid-specified because rifampin also quadrupled transfer to UV-irradiated recipients which were deficient in the transcription of the resident DNA. Successive strands of ColI appear to be transferred discontinuously, because the transferred DNA accumulated in normal and rifampin-treated recipients in the form of circular and linear monomeric units. Although rifampin treatment of recipients also increased transfer of a second Ialpha plasmid, R144drd-3, by about four times, the drug failed to cause a substantial increase of Flac transfer in comparable matings.

In vivo transcription of R-plasmid deoxyribonucleic acid in Escherichia coli strains with altered antibiotic resistance levels and/or conjugal proficiency

The amounts of plasmid deoxyribonucleic acid (DNA) and the levels of the in vivo transcription of the Escherichia coli plasmids R538-1 (repressed for conjugal transfer) and R538-1drd (derepressed for transfer) were determined by DNA-DNA hybridization and DNA-ribonucleic acid hybridization, respectively. The results demonstrate that the level of plasmid transcription is increased by two-fold in the strain carrying the derepressed plasmid, compared to an isogenic strain carrying the repressed plasmid, whereas the amount of plasmid DNA is approximately the same, suggesting that the transfer genes are under transcriptional control. Levels of plasmid DNA, plasmid DNA transcription, and chloramphenicol acetyltransferase activity were also compared in a mutant strain that carried the R538-1drd plasmid and was resistant to high levels of antibiotics. This strain produces about 13 copies of plasmid DNA per chromosome compared to five copies for the parent strain. The level of transcription of plasmid DNA was found to be twofold higher in the high-level resistant strain, whereas the level of chloramphenition, acetyltransferase activity was increased by 10-fold. In addition the levels of plasmid DNA transcription and chloramphenicol acetyltransferase activity in the high-level resistant strain were found to be further increased by the presence of high levels of chloramphenicol in the growth medium. The amount of plasmid DNA remained constant under these conditions, indicating that high levels of chloramphenicol can stimulate the expression of plasmid genes at the level of transcription in this strain.

Extrachromosomal deoxyribonucleic acid in R factor-harboring Enterobacteriaceae

Extrachromosomal deoxyribonucleic acid (DNA) from 24 different R factor-harboring Enterobacteriaceae was isolated and characterized by analytical ultracentrifugation and electron microscopy. The R factors represented 15 different patterns of transferable drug resistance found in enterobacteria from an enclosed geographic area. All of the strains contained extrachromosomal, circular DNA molecules within the range of 0.4 to 52 mum. More than one size class of circular DNA molecules was observed in the majority of the extrachromosomal DNA preparations. The buoyant density of the extrachromosomal DNA ranged from 1.700 to 1.720 g/cm3. The majority of the bacteria contained extrachromosomal DNAs of various densities. Three-fourths of the R factors were classified as fi+. The investigation illustrates the extensive variability in the physical characteristics of plasmid DNA from R factor-harboring strains.

Integrative suppression of a dnaA mutation in Salmonella typhimurium

Integrative suppression of a dnaA mutation in Salmonella typhimurium may result from the integration of F'lac or F'his into the chromosome in the left hand side of the chromosomal map. The suppressed revertants resulting from this integration do not contain DNA of the F' elements in the covalently closed circular (CCC)1 form but still contain the CCC DNA of the cryptic LT2 plasmid. Two suppressed revertants isolated from dnaA/F- strains were found in which the suppression of dnaA character was accompanied by the loss of CCC DNA from the cell lysates. From one of these revertants a segregant was isolated in which the return to the dnaA phenotype was accompanied by the reappearance of CCC DNA in the cell lysate. It is suggested that the cryptic plasmid may integrate into the chromosome of S. typhimurium and this integration may result in suppression of the dnaA mutation. Additional evidence suggesting that the cryptic plasmid controls its own initiation of replication independently of the function of the chromosomal dnaA gene is supplied by the results of the determination of incorporation of labelled thymidine into CCC DNA of the dnaA1 strain at the nonpermissive temperature.

Partial suppression of the phenotype of Escherichia coli K-12 dnaG mutants by some I-like conjugative plasmids

Three I-like conjugative plasmids, ColIdrd1, R144drd3, and R64drd11, which are derepressed for functions involved in conjugation, were found to suppress at least partially the phenotype of temperature-sensitive dnaG mutants of Escherichia coli K-12, as judged from the kinetics of deoxyribonucleic acid synthesis at elevated temperature in newly formed and established plasmid-containing strains. In contrast, the corresponding wild-type plasmids and three F-like derepressed conjugative plasmids, F101, R100drd1, and R1drd16, all failed to suppress. Suppression is presumably caused by a different plasmid-determined function from that which promotes survival of ultraviolet-irradiated bacteria, because both the wild-type I-like plasmids and their drd mutants protected irradiated bacteria. One possible interpretation of these results is that the product of a gene carried by certain I-like plasmids can substitute for the bacterial dnaG gene product during ongoing deoxyribonucleic acid replication.

Asymmetry and extent of in vivo transcripition of R-plasmid deoxyribonucleic acid in Escherichia coli

Deoxyribonucleic acid-ribonucleic acid (DNA-RNA) hybridization studies have been performed with R-plasmid DNA (R538-1drd) and in vivo-synthesized RNA. R-plasmid DNA was isolated from Escherichia coli K-12, and the complementary strands were separated in cesium chloride-polyuridylic acid-polyguanylic acid gradients. DNA-RNA hybridization was performed with the separated DNA strands and RNA purified from R-plasmid-carrying cells. The results demonstrated that an asymmetric transcription of the R-plasmid DNA occurs in vivo. Hybridization was only detected with the H strand (denser strand in cesium chloride-polyuridylic acid-polyguanylic acid). By determining the density of the RNA-DNA hybrid in CsCl gradients, it was estimated that greater than 60% of the nucleotide sequences in the R-plasmid DNA are transcribed in logarithmically growing E. coli cells. No R-plasmid-specific RNA was detected in E. coli cells that did not carry the plasmid.

Properties of the relaxation complexes of supercoiled deoxyribonucleic acid and protein of the R plasmids R64, R28K, and R6K

The presence of supercoiled deoxyribonucleic acid in the form of a relaxation complex is described for the antibiotic resistance plasmids R64, R28K, and R6K. The properties of these relaxation complexes indicate that they consist of a covalently closed circular deoxyribonucleic acid molecule associated with an activable, single strand-specific endonuclease.

Evidence for a composite state of an F'his, gnd element and a cryptic plasmid in a derivative of Salmonella typhimurium LT2

A method designed to select mutants constitutive for expression of the histidine operon has been applied to a Salmonella typhimurium LT2 strain containing an F'his,gnd element and a cryptic plasmid. One of the mutants isolated, strain AA0019, has not only increased levels of histidinol phosphate phosphatase (hisB), but also increased levels of gluconate-6-phosphate dehydrogenase (gnd). Ultracentrifugation studies of extrachromosomal deoxyribonucleic acid (DNA) isolated from strain AA0019 revealed the presence of a single species of covalently closed circular (CCC) DNA that sedimented more rapidly through neutral and alkaline sucrose gradients than any of its possible plasmid precursors. From neutral sucrose gradients, sedimentation coefficients of 130, 100, and 86S were derived, corresponding to the CCC DNA of the large plasmid in strain AA0019, the F'his,gnd element and the cryptic LT2 plasmid, respectively. An Escherichia coli plasmid-free strain that upon mating had received the large 130S plasmid also contained 86S and 100S CCC DNA components. A histidine-requiring derivative of strain AA0019 obtained after acridine orange treatment retained the cryptic plasmid DNA. Apparently, the large plasmid in strain AA0019 consists of the F'his,gnd element and the cryptic LT2 plasmid of the parental strain.

Molecular nature of two nonconjugative plasmids carrying drug resistance genes

Two nonconjugative R-plasmids, N-SuSm and N-Tc, have been characterized. Both were of relatively small size (5 x 10(6) to 6 x 10(6) daltons) and present in multiple copies within their respective bacterial hosts. N-SuSm possessed a guanine plus cytosine content of 55%, whereas N-Tc was 49% guanine plus cytosine. Although these plasmids were inherently nontransmissible they could be mobilized by a large variety of transfer agents including Ent, Hly, and K88. The fi(-) transfer factors tested were far more likely (about 200x) to mobilize these nonconjugative plasmids than were the fi(+) transfer factors tested. Although the mobilization phenomenon was not found to be associated with a detectable level of direct stable recombinational union between N-SuSm or N-Tc with a transfer factor, we were able to demonstrate a low level of recombination between these replicons and a transfer factor by P1-mediated transduction. The isolation of recombinants between transfer factors and nonconjugative plasmids presumably represents one means by which unitary molecular types of R-plasmids arise and by which existing R-plasmids may acquire new resistance determinants.

Conjugal deoxyribonucleic acid replication by Excherichia coli K-12: effect of chloramphenicol and rifampin

Conjugal replication of R64-11 deoxyribonucleic acid (DNA) and the concomitant transfer of R64-11 DNA to DNA-deficient minicells are dependent upon processes that are inhibited by rifampin and chloramphenicol. The rifampin-sensitive product is not present in vegetatively growing cells and is needed to initiate both conjugal DNA replication in donor cells and DNA transfer to recipient minicells. If the rifampin-sensitive product is a ribonucleic acid (RNA) molecule (rather than RNA polymerase itself), our data indicate that this RNA species required for initiation of conjugal activity does not need to be translated into a protein product. The chloramphenicol-sensitive product(s) is present in vegetatively growing cells in sufficient quantity to permit most donor cells to carry out one round of plasmid conjugal replication and transfer. The initiation of second and subsequent rounds of conjugal replication and transfer are dependent on the synthesis of both the rifampin-sensitive and chloramphenicol-sensitive products. Our results demonstrate a correspondence between the amount of conjugal DNA replication in the donor and the amount of DNA transferred to recipient minicells under all conditions, and therefore suggest but do not prove that plasmid transfer is dependent on conjugal DNA replication. The results also add additional proof that R64-11 transfer to minicells is discontinuous. All of these results are discussed in regard to further refinements of old models for the mechanism of conjugal transfer as well as a more radical departure from current dogma.

Conjugal deoxyribonucleic acid replication by Escherichia coli K-12: effect of nalidixic acid

During the conjugal transfer of the R64-11 plasmid at 42 C from donor cells thermosensitive for vegetative deoxyribonucleic acid (DNA) synthesis to recipient minicells, the plasmids are conjugally replicated in the donor cells. This conjugal replication is inhibited by nalidixic acid, and the degree of inhibition is comparable to the reduction in the amount of plasmid DNA transferred to the recipient minicells in the presence of the drug. In addition, the size of DNA transferred to the minicells and the fraction of conjugally replicated DNA in the donor cells that can be isolated as closed-circular plasmid DNA under alkaline conditions are both reduced by nalidixic acid. When the drug is added to a mating that is underway, the rate of conjugal replication is immediately reduced. This change is accompanied by a reduction in the amount of conjugally replicated DNA in the donor cells that can be isolated as closed-circular plasmid DNA. Furthermore, conjugally replicated plasmid DNA that is not associated with the donor cell membrane becomes membrane bound after the addition of nalidixic acid.

Conjugal deoxyribonucleic acid replication by Escherichia coli K-12: stimulation in dnaB(ts) donors by minicells

R64-11(+) donor cells that are thermosensitive for vegetative DNA replication will synthesize DNA at the restrictive temperature when recipient minicells are present. This is conjugal DNA replication because it is R64-11 DNA that is being synthesized and there is no DNA synthesis if minicells that cannot be recipients of R64-11 DNA are used. The plasmid DNA present in the donor cells before mating is transferred to recipient minicells within the first 20 min of mating, but additional copies of plasmid DNA synthesized during the mating continue to be transferred for at least 90 min. However, the transfer of R64-11 DNA to minicells is not continuous because the plasmid DNA in minicells is the size of one R64-11 molecule or smaller, and there are delays between the rounds of plasmid transfer. DNA is synthesized in minicells during conjugation, but this DNA has a molecular weight much smaller than that of R64-11. Thus, recipient minicells are defective and are not able to complete the synthesis of a DNA strand complementary to the single-stranded R64-11 DNA received from the donor cell.

Plasmid in Bacillus pumilus and the enhanced sporulation of plasmid-negative variants

Approximately 3% of the deoxyribonucleic acid (DNA) of Bacillus pumilus NRS576 can be isolated as covalently closed, circular duplex molecules of homogeneous size. The mol wt of the 576 plasmid is approximately 30 million. NRS576 (plasmid(+)) is oligosporogenic; less than 1% of the cells form spores during incubation in liquid AK sporulation medium. Variants that form spores at a high frequency (greater than 50% spores in 24 h) occur spontaneously at a frequency of approximately 10(-5). More than 25 such variants have been examined and all lack detectable plasmid DNA. A relationship appears to exist between the oligosporogenic properties of NRS576 and the presence of the 576 plasmid.

Extrachromosomal deoxyribonucleic acid in different enterobacteria

Eighty-seven different enterobacteria and pseudomonas strains were examined for the presence of extrachromosomal deoxyribonucleic acid (DNA). Thirty-four strains contained closed circular DNA by the ethidium bromide CsCl density technique. Extrachromosomal DNA was most frequent in Escherichia and Klebsiella strains. The extrachromosomal DNA was isolated and characterized by analytical ultracentrifugation and electron microscopy. All the extrachromosomal DNA-containing bacteria contained circular DNA molecules of small size (0.5-4 mum). Most of these bacteria also contained larger circles (20-40 mum). The number of different size classes of circular DNA in each strain varied from one to five. The buoyant density of the extrachromosomal DNA ranged from 1.692 to 1.721 g/cm(3). Many bacteria contained extrachromosomal DNA of more than one density.

Molecular structure of an R factor, its component drug-resistance determinants and transfer factor

Plasmid DNA from Escherichia coli strains harboring drug resistance either of the infectious or noninfectious kind has been separated by CsCl centrifugation of crude cell lysates in the presence of ethidium bromide and examined by electron microscopy. Plasmid deoxyribonucleic acid (DNA) from an S(+) strain (which has the property of noninfectious streptomycin-sulfonamide resistance) consists of a monomolecular covalently closed circular species of 2.7 mum in contour length (5.6 x 10(6) atomic mass units; amu). DNA from a strain carrying a transfer factor, termed Delta, but no determinant for drug resistance, is a monomolecular covalently closed circular species of 29.3 mum in contour length (61 x 10(6) amu). DNA from either Delta(+)A(+) or Delta(+)S(+) strains, (which are respectively ampicillin or streptomycin-sulfonamide resistant, and can transfer this drug resistance), shows a bimodal distribution of molecules of contour lengths 2.7 mum and 29.3 mum, whereas DNA from a (Delta-T)(+) strain (showing infectious tetracycline resistance) contains only one species of molecule measuring 32.3 mum (67 x 10(6) amu). We conclude that ampicillin resistance is carried by a DNA molecule (the A determinant) of 2.7 mum, and streptomycin-sulfonamide resistance is carried by an independent molecule (the S determinant) of similar size. These molecules are not able to effect their own transfer, but can be transmitted to other cells due to the simultaneous presence of the transfer factor, Delta, which also constitutes an independent molecule, of size 29.3 mum. In general, there appears to be little recombination or integration of the A or S molecules into that of Delta, although a small proportion (5-10%) of recombinant molecules cannot be excluded. In contrast, the third drug-resistance determinant, that for tetracycline resistance (denoted as T), is integrated in the Delta molecule to form the composite structure Delta-T of size 32.3 mum, which determines infectious tetracycline resistance. The Delta(+)A(+) and Delta(+)S(+) strains are defined as harboring plasmid aggregates, and the (Delta-T)(+) strain is defined as carrying a plasmid cointegrate; the properties of all three strains are characteristic of strains harboring R factors. These results are compatible with the previously published genetic data. The number of Delta molecules per cell appears to be equal to the chromosomal number irrespective of growth phase, and this plasmid can thus be defined as stringently regulated in DNA replication. In contrast, S and A exist as multiple copies, probably in at least a 10-fold excess of chromosomal copy number. S and A can thus be defined as relaxed in the regulation of their DNA replication.

Plasmid-associated functions of a stable Flac

Several functions associated with the stable plasmid, FlacS, have been examined. Our results indicate that the sex pili synthesized by Escherichia coli strains carrying FlacS are altered in some manner as evidenced by a very inefficient adsorption of male-specific phages. On the other hand, FlacS-mediated entry exclusion of related plasmids and plasmid incompatibility function(s) appear normal. The presence of covalently closed circular deoxyribonucleic acid in E. coli strains harboring FlacS indicates that it is an autonomously replicating plasmid. Based on beta-galactosidase levels and the percentage of covalently closed circular deoxyribonucleic acid, it appears that the stability of the FlacS is not the result of multiple copies of this plasmid. FlacS appears larger than its precursor, F(ts114)lac, in sedimentation through alkaline sucrose gradients.

Molecular structure of bacterial plasmids

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Characterization by electron microscopy of fused F-prime factors in Escherichia coli

During previous attempts to find mutants in which incompatibility between two F-prime (F') factors of E. coli is abolished, we found that two F' factors harbored in the same cell occasionally recombined to form a single genetic unit. Thus, a fused F'trp(+)arg(+) factor was obtained from a mating of a strain carrying recA F'trp(+) with a strain carrying recA F'arg(+). In the present paper physical fusion of the two F' factors, inferred from genetic studies, is confirmed by electron microscopy. F DNA was extracted, after detergent lysis, from four fused F' strains: the parental F'trp(+) and F'arg(+) strains and three F(+) strains. This supercoiled DNA was separated on ethidium bromide-cesium chloride gradients and converted to the open circular from before electron microscopy. The contour lengths in mum of the various DNA preparations were: F'trp(+), 59.4; F'arg(+), 33.2; the four F'trp(+)arg(+) factors, 38.5, 39.6, 63.2, and 73.5: the three F factors, 31.1, 31.1, and 31.0. Thus, different fused F' factors formed from the same parental F' factors vary in length, but are shorter than the sum of the parental F' factors.

Photoreactivatiion, excision, and strand-rejoining repair in R factor-containing minicells of Escherichia coli K-12

Chromosomeless "minicells" are formed by misplaced cell fissions near the polar extremities of an Escherichia coli K-12 mutant strain. Resistance (R)-factor deoxyribonucleic acid (DNA) can be introduced into minicells by segregation from an R(+) (R64-11) derivative of the original mutant. We have assessed the ability of R(+) minicells to correct defects produced in their plasmid DNA by ultraviolet (UV) and gamma radiations. Minicells harboring plasmid DNA, in comparison with their repair-proficient minicell-producing parents, possess (i) an equal competence to rejoin single-strand breaks induced in DNA by gamma rays, (ii) a reduced capacity for the photoenzymatic repair of UV-induced pyrimidine dimers, and (iii) a total inability to excise dimers, apparently owing to a deficiency in UV-specific endonuclease activity responsible for mediating the initial incision step in excision repair. Assuming that the DNA repair properties of R(+) minicells reflect the concentration of repair enzymes located in the plasmid-containing polar caps of entire cells, these findings suggest that: (i) the enzymes responsible for rejoining single-strand breaks are distributed throughout the cell; (ii) photoreactivating enzyme molecules tend to be concentrated near bacterial DNA and to a lesser extent near plasmid DNA; and (iii) UV-specific endonuclease molecules are primarily confined to the central region of the E. coli cell and, thus, seldom segregate with R-factor DNA into minicells.