Replication of the minicircular DNA of E. coli 15 is dependent on DNA polymerase I but independent of DNA polymerase 3

Regeneration of insertionally inactivated streptococcal DNA fragments after excision of transposon Tn916 in Escherichia coli: strategy for targeting and cloning of genes from gram-positive bacteria

The conjugative transposon Tn916 (15 kilobases), originally identified in Streptococcus faecalis DS16, has been cloned as an intact element on the pBR322-derived vector pGL101 in Escherichia coli. The EcoRI F' (EcoRI F::Tn916) fragment of pAM211 (pAD1::Tn916) was cloned into the single EcoRI site of pGL101 to form the chimera, pAM120, by selecting for the expression of Tn916-encoded tetracycline resistance (Tcr). Interestingly, in the absence of continued selection for Tcr, Tn916 excised from pAM120 at high frequency. This excision event resulted in a plasmid species consisting of the pGL101 vector and a 2.7-kilobase restriction fragment comigrating with the EcoRI F fragment of pAD1 during agarose gel electrophoresis. Filter blot hybridization experiments showed the 2.7-kilobase fragment generated as a result of Tn916 excision to be homologous with the EcoRI F fragment of pAD1. Analogous results were obtained with another chimera, pAM170, generated by ligating the EcoRI D' (EcoRI D::Tn916) fragment of pAM210 (pAD1::Tn916) to EcoRI-digested pGL101. Comparison of the AluI and RsaI cleavage patterns of the EcoRI F fragment isolated after Tn916 excision with those from an EcoRI F fragment derived from pAD1 failed to detect any difference in the two fragments: data in support of a precise Tn916 excision event in E. coli. Subcloning experiments showed that an intact transposon was required for Tn916 excision and located the Tcr determinant near the single HindIII site on Tn916. Although excision occurred with high frequency in E. coli, Tn916 insertion into the E. coli chromosome was a much rarer event. Tcr transformants were not obtained when pAM120 DNA was used to transform a polA1 strain, E. coli C2368.

Replication of the mini-R1 plasmid Rsc11 and Rsc11 hybrid plasmids

Replication of the multicopy mini-R1 plasmid, Rsc11, is dependent on host replication functions dna A, B, C, E and G but independent of polA1. Chloramphenicol immediately stops its replication. A stable relaxation complex is not formed. Composite plasmids were constructed with Rsc11 and other small replicons like pSC101, ColE1 and mini-ColE1. In all combinations the amount of hybrid plasmid DNA in the cell never exceeds the amount of Rsc11 DNA itself. This leads to varying copy numbers of the hybrid plasmids depending on the size of the second plasmid. Replication of the composite plasmids proceeds probably always under the control of the Rsc11 part although the second replicon is still functional. The composite plasmids are incompatible with both the parent replicons.

REPLICAtion of small plasmids in extracts of Escherichia coli: requirement for both DNA polymerases I and II

The role of the three E. coli DNA polymerases (pol I, II, and III) in the replication of Col E1 DNA and other small plasmids with similar replicative properties was investigated in a soluble in vitro system prepared by freeze-thaw lysis of chloramphenicol-treated cells (Staudenbauer, 1976). Extracts from isogenic mutants of the polA, polB and polC gene loci deficient in pol I, II, and III respectively were examined for their replicative capacity. It was found that polA and polC extracts are deficient in the synthesis of supercoiled plasmid DNA, whereas the polB mutation has not effect. Deficient extracts could be complemented by addition of purified pol I and pol III holoenzyme. Analysis of the in vitro synthesized DNA by alkaline gradient centrifugation indicates that pol I is involved in an early step of the replication cycle whereas pol III is required at a later stage. These conclusions are confirmed by inhibition studies employing arabionsylcytosine triphosphate (aCTP) which is shown to interfere with pol III as well as pol II. The strong inhibitory effect of aCTP on plasmid replication is not influenced by the polB mutation and mimicks the effects of thermal inactivation of polC extracts. It is suggested that aCTP blocks plasmid ENA replication in vitro by interfering with pol III function

Characterization of a replication mutant of the bacteriocinogenic plasmid Clo DF13

In a previous paper (Kool, A.J. and Nijkamp, H.J.J. (1974) J. Bacteriol. 120, 569--578) the isolation of a mutant of the bacteriocinogenic plasmid Clo DF13-Rep3, has been described. It was observed that cells harbouring the wild type plasmid synthesize more plasmid DNA cells harbouring the wild type plasmid. This paper deals with the characterization of the nature of this plasmid-specific mutation. The following properties of the Clo DF13-Rep3 mutant plasmid could be observed: 1. The plasmid-specific mutation did not lead to a significant change in the sedimentation value of Clo DF13 DNA. 2. The specific rate of Clo DF13-Rep3 DNA synthesis (expressed as the number of plasmid DNA molecules synthesized per min) is on average seven times the specific rate of wild type Clo DF13 DNA synthesis. 3. Also chromosomeless minicells, harbouring the Clo DF 13-Rep 3 plasmid, contain about seven times more plasmid DNA as wild type Clo DF13 harbouring minicells. 4. The replication time of the Clo DF13-Rep3 plasmid is approx. 90 s at 30 degrees C and does not differ significantly from the replication time of the wild type plasmid. 5. The Rep3 mutation did not alter the dependence of Clo DF13 plasmid replication on the dnaA and dnaC gene products. 6. The plasmid-specific mutation is cis-dominant over wild type. The data presented in this paper indicate that this mutant plasmid is not affected in the elongation but in the initiation of plasmid DNA replication.

Characterization and replication control of plasmids from Enterobacter cloacae DF 13

It has been shown previously that Enterobacter clocacae DF13 harbours at least five different size classes of plasmids. A 45 x 10(6)-Mr self-transmissible R factor determining resistance against tetracyclin, sulfanilamide, streptomycin and chloramphenicol, a 6.0 x 10(6)-Mr bacteriocinogenic factor without sex factors activity and cryptic plasmids in the size classes of Mr 1.3 x10(6), 2.8 x 10(6) and 8.0 x 10(6) respectively. The present work deals with the determination of the homogeneity and molecular relationship of 1.3 x 10(6)-Mr (mini) and 2.8 x 10(6)-Mr (midi) cryptic plasmids and the 6.0 x 10(6)-Mr (maxi) bacteriocinogenic factor, their kinetics of replication and their replication control in response to inhibition of protein synthesis.

Effects of different alleles of the E. coli K12 pol A gene on the replication of non-transferring plasmids

The effects of eight different polA alleles on the replication of six different non-transferring enterobacterial plasmids have been tested. Using phage P1CM transduction, different allelic polA mutations were introduced into E. coli K12 strains carrying one of several antibiotic resistance plasmids. Plasmid stability in the transductants was examined by testing clones for drug resistance after growth under various conditions. From the results, the R factors may be divided into three different classes. One plasmid is only affected by PolA conditions which inhibit host cell growth, 3 plasmids (from the same compatibility group) are unstable under conditions in which the cells are severely deficient in DNA polymerase I and two other plasmids (compatible with each other and with the other 4) are immediately lost from such transductants and are unstable in a number of others. Furthermore, the plasmids which are most dependent on DNA polymerase I have been shown to replicate in the presence of chloramphenicol and therefore typigy a class of plasmids which includes bacteriocinogenic factors such as ColE1 and CloDF13, resistance determinant RSF1030 and the E. coli 15 minicircular plasmid.

Increased spontaneous reversion of certain frameshift mutations in DNA polymerase I deficient strains of Escherichia coli

A tenfold increase in the spontaneous reversion frequency of two of six lacZ frameshift mutations tested was observed in strains containing the following DNA polymerase I mutations--polA1, polA5, polA6, polAex1, res-3 and resA1. Reconstruction experiments indicated that this increase was not the result of a selective effect. Only a fourfold increase in frameshift mutations was found in strains containing a polA107 mutation. Both the polAex1 and polA107 mutations result in defective 5' to 3' exonuclease activity and do not affect polymerizing activity, but have different effects on frameshift mutation. A polA mutation on the chromosome induced frameshift mutations in a gene on an F episome. None of three auxotrophic mutations studied showed high frequency reversion in the presence of the polA1 or polA6 mutations.

Binding of bacteriocin Clo DF13 to Clo DF13 plasmid deoxyribonucleic acid in vivo and in vitro

The bacteriocinogenic plasmid Clo DF13 is present in Escherichia coli to the extent of 10 copies per cell. A complex of Clo DF13 plasmid deoxyribonucleic acid (DNA) and protein can be isolated from cells. Treatment of the complex with ionic detergents or proteases dissociates the complex but does not convert any supercoiled Clo DF13 DNA to the open circular form, indicating that this complex is not a relaxation complex. The complex is stable in 0.5 M NaCl and contains one polypeptide species. The protein, present in the complex, appeared to be bacteriocin Clo DF13 for the following reasons: (i) the protein is de novo synthesized in Clo DF13-harboring minicells, indicating that this protein is Clo DF13 specific; (ii) this protein shows bacteriocinogenic activity on a bacteriocin Clo DF13-susceptible indicator strain; (iii) this protein has the same molecular weight (60,000) as bacteriocin Clo DF13. DNA-protein binding experiments, involving QAE-Sephadex column chromatography and nitrocellulose membrane filters, demonstrate that bacteriocin Clo DF13 has also affinity in vitro for Clo DF13 DNA. Membrane filter binding experiments revealed that bacteriocin Clo DF13 does not interact with other DNA species, such as ColE1 DNA, yeast DNA, calf thymus DNA, phiX174 DNA, and also not with denatured Clo DF13 DNA. In addition no binding to Clo DF13 DNA of a related bacteriocin, colicin E3, could be detected. These results indicate that the binding of bacteriocin Clo DF 13 to double-stranded Clo DF13 DNA is very specific.

R6K plasmid replication: influence of chromosomal genotype in minicell-producing strains of Escherichia coli K-12

Alkaline sucrose velocity sedimentation and cesium chloride-ethidium bromide equilibrium centrifugation have been used to determine the number of copies per chromosomal equivalent of the relaxedly replicating R6K plasmid (a conjugative plasmid conferring ampicillin and streptomycin resistance) in two minicell-producing strains of Escherichia coli K-12. In one strain, the average number of covalently closed circular R6K molecules per chromosomal equivalent is 13 in log-phase and 35 in stationary-phase cells. In the other strain, there is an average of six covalently closed circular R6K molecules per chromosomal equivalent in both log- and stationary-phase cells. Selection from this strain of spontaneously occurring mutants resistant to high concentrations of ampicillin has been accomplished and such mutants show a two- to threefold increase in the number of R6K copies per chromosomal equivalent. Relative to the parental strain, mutants display the following properties: (i) elevated streptomycin resistance, (ii) a 10-fold increase in R6K conjugal transfer, (iii) a 10-fold increase in the amount of R6K plasmid deoxyribonucleic acid segregated into minicells, and (iv) a two- to threefold increase in R6K-specified beta-lactamase. The mutation(s) responsible for the increase in the number of R6K molecules per chromosomal equivalent is located on the bacterial chromosome. No R6K-linked mutations conferring the above phenotypes have been obtained. The mutations are presumed to be in chromosomal genes which play a role in the regulation of R6K replication in this strain.

Replication of the bacteriocinogenic plasmid Clo DF13 in thermosensitive Escherichia coli mutants defective in initiation or elongation of deoxyribonucleic acid replication

The replication of the bacteriocinogenic plasmid Clo DF13 has been studied in the seven temperature-sensitive Escherichia coli mutants defective in deoxyribonucleic acid (DNA) replication (dnaA-dnaG). Experiments with dna initiation mutants revealed that the replication of the Clo DF13 plasmid depends to a great extent on the host-determined dnaC (dnaD) gene product, but depends slightly on the dnaA gene product. The synthesis of Clo DF13 plasmid DNA also requires the dnaF and dnaG gene products, which are involved in the elongation of chromosomal DNA replication. In contrast, the Clo DF13 plasmid is able to replicate in the dnaB and dnaE elongation mutants at the restrictive temperature. When de novo protein synthesis is inhibited by chloramphenicol in wild-type cells, the Clo DF13 plasmid continues to replicate for at least 12 h, long after chromosomal DNA synthesis has ceased, resulting in an accumulation of Clo DF13 DNA molecules of about 500 copies per cell. After 3 h of chloramphenicol treatment, the Clo DF13 plasmid replicates at a rate approximately five times the rate in the absence of chloramphenicol. Inhibition of protein synthesis by chloramphenicol does not influence the level of Clo DF13 DNA synthesis at the restrictive temperature in the dna mutants, except for the dnaA mutant. Chloramphenicol abolishes the inhibition of Clo DF13 DNA synthesis in the dnaA mutant at the nonpermissive temperature. Under these conditions, Clo DF13 DNA synthesis was slightly stimulated in the first 30 min after the temperature shift, and continued for more than 3 h at an almost uninhibited level.

Utilization of two distinct modes of replication by a hybrid plasmid constructed in vitro from separate replicons

A hybrid plasmid, pSC134, that codes for two distinct sets of replication functions has been constructed in vitro by ligation of EcoRI endonuclease-cleaved pSC101 and Col E1 plasmid replicons, and has been introduced into Escherichia coli by transformation. The replication properties of the pSC134 plasmid in DNA polymerase I-defective mutants or in the presence of chloramphenicol indicate that this hybrid plasmid can utilize the functionally distinct modes of replication specified by both of its parent replicons.

Influence of protein and ribonucleic acid synthesis on the replication of the bacteriocinogenic factor Clo DF13 in Escherichia coli cells and minicells

The influence of ribonucleic acid (RNA) and protein synthesis on the replication of the cloacinogenic factor Clo DF13 was studied in Escherichia coli cells and minicells. In chromosomeless minicells harboring the Clo DF13 factor, Clo DF13 deoxyribonucleic acid (DNA) synthesis is slightly stimulated after inhibition of protein synthesis by chloramphenicol or puromycin and continues for more than 8 h. When minicells were treated with rifampin, a specific inhibitor of DNA-dependent RNA polymerase, Clo DF13 RNA and DNA synthesis appeared to stop abruptly. In cells, the Clo DF13 factor continues to replicate during treatment with chloramphenicol long after chromosomal DNA synthesis ceases. When rifampin was included during chloramphenicol treatment of cells, synthesis of Clo DF13 plasmid DNA was blocked completely. Isolated, supercoiled Clo DF13 DNA, synthesized in cells or minicells in the presence of chloramphenicol, appeared to be sensitive to ribonuclease and alkali treatment. These treatments convert a relatively large portion of the covalently closed Clo DF13 DNA to the open circular form, whereas supercoiled Clo DF13 DNA, isolated from non-chloramphenicol-treated cells or minicells, is not significantly affected by these treatments. These results indicate that RNA synthesis and specifically Clo DF13 RNA synthesis are involved in Clo DF13 DNA replication and that the covalently closed Clo DF13 DNA, synthesized in the presence of chloramphenicol, contains one or more RNA sequences. De novo synthesis of chromosomal and Clo DF13-specific proteins is not required for the replication of the Clo DF13 factor. Supercoiled Clo DF13 DNA, isolated from a polA107 (Clo DF13) strain which lacks the 5' --> 3' exonucleolytic activity of DNA polymerase I, is insensitive to ribonuclease or alkali treatment, indicating that in this mutant the RNA sequences are still removed from the RNA-DNA hybrid.