Circular R-factor molecules controlling penicillinase synthesis, replicating in Escherichia coli under either relaxed or stringent control

Gene lon and plasmid inheritance in Escherichia coli K-12

Lon- mutants of Escherichia coli K-12 are deficient in the inheritance of F-plasmids by conjugation. This deficiency is distinct from the conjugation deficiency caused by overproduction of capsular polysaccharide which decreases donor-recipient pair formation.

Penicillin selection of Escherichia coli deoxyribonucleic acid repair mutants

A rapid method has been developed for isolation of ultraviolet-sensitive mutants of Escherichia coli, by inducing delay in the growth and/or division of repair-deficienct cells with low fluences of far-ultraviolet radiation, and killing with penicillin the repair-proficient cells, which continue to grow and divide. With this technique, we have achieved about a 3,000-fold enrichment for photoreactivation less (phr) cells and have isolated and characterized three phr mutants.

Nucleotide sequence of the region of an origin of replication of the antibiotic resistance plasmid R6K

A 2.1-kilobase segment of the antibiotic resistance plasmid R6K carries sufficient information to replicate as a plasmid in Escherichia coli. This segment contains a functional origin of replication and a structural gene for a protein, designated pi, that is required for the initiation of R6K replication. The nucleotide sequence of a 520-base-pair portion of this 2.1-kilobase segment that includes the functional origin of replication and the region adjacent to the start of the pi structural gene was determined. A striking feature of the sequence is the presence of seven 22-base-pair direct repeats joined in tandem in the region adjacent to the start of the pi gene. A possible role of the tandem repeats in the regulation of expression of the pi protein and the control of initiation of replication of the plasmid R6K is discussed.

Trans-complementation-dependent replication of a low molecular weight origin fragment from plasmid R6K

A non-self-replicating segment (1370 base pairs) of plasmid R6K was cloned in E. coli and shown to trans-complement temperature-sensitive replication mutants of this plasmid. This segment contains the gene which codes for a protein required for initiation of replication of the plasmid, and was used as a helper in a functional assay for an origin of replication in R6K derivatives. A 420 bp fragment, derived from R6K DNA, was shown to carry a functional origin since it was capable of replicating as a plasmid in E. coli cells carrying the helper segment either on the host chromosome or on a plasmid Col E1 derivative. The copy number of the origin fragment in cells carrying the helper segment on the chromosome is essentially the same as the copy number of R6K. A model for the positive regulation of plasmid R6K replication is presented.

Requirement of a plasmid-encoded protein for replication in vitro of plasmid R6K

Conditions are described for the replication of exogeneous R6K DNA in an in vitro system prepared from Escherichia coli cells. Replication of plasmid DNA in this system is semiconservative and sensitive to actinomycin D, novobiocin, arabinofuranosyl-CTP,N-ethylmaleimide, and inhibitors of DNA-dependent RNA polymerase. An ammonium sulfate fraction prepared from cells carrying the R6K plasmid is required for replication. A direct role in replication for a plasmid-encoded protein, designated pi, in this fraction is indicated by the inactivity of this fraction when prepared from cells carrying a temperature-sensitive mutant plasmid and the thermolability of this fraction when prepared from cells carrying a partial revertant of the mutant plasmid. This plasmid-encoded protein is necessary for the initiation of R6K DNA replication and functions before or during the formation of nascent RNA in the initiation process. The results of titration assays of this protein using various template DNAs suggest that the protein interacts with the plasmid DNA at the region essential for DNA replication.

Characterization of a bacteriocinogenic plasmid in Clostridium perfringens CW55

A bacteriocinogenic strain of Clostridium perfringens was exposed to various curing agents known to accelerate the elimination of extrachromosomal DNA, and 20 independently derived mutants that had lost both the ability to produce bacteriocin and their immunity to it were isolated and characterized. All of the mutants were missing at least two specific plasmid bands seen in the agarose gel plasmid profile of the parent strain. Evidence that the two missing bands represented the open circular and closed circular forms of the same plasmid was obtained by X-ray nicking and restriction endonuclease digestion. The data indicated that bacteriocin production and immunity are controlled by a single plasmid, pCW4, with a molecular weight of 5.6 x 10(6) in this strain. Attempts to transfer the bacteriocinogenic plasmid were unsuccessful.

Interaction between the plasmid R6K and Escherichia coli with defective DNA polymerase I

Initial experiments demonstrated that the plasmid R6K cannot be transferred to or maintained readily in theE. coliDNA polymerase I deficient strain JG138polA1. Results withE. coliMM386polA12(R6K), which has a temperature sensitive polymerase I enzyme, showed cell division becomes abnormal when the polymerase I enzyme of the host bacteria is inactivated at the restrictive temperature. Under conditions of polymerase I deficiency, R6K replication, as measured by monitoring R-factor-mediated β-lactamase activity, also becomes abnormal with the loss of multiple R6K copies per cell and the apparent maintenance of a single R-factor copy per cell.

Relationship of R6K replicating forms to the folded chromosome of Escherichia coli

An examination of the relationship of both nonreplicating and replicating forms of R6K plasmid DNA to the Escherichia coli folded chromosome showed that both forms cosediment with the chromosome in neutral sucrose gradients. Approximately 20% of the nonreplicatin molecules was found as freely sedimenting forms when the folded-configuration of the chromosomes was preserved. However, under the same conditions negligible amounts of the replicating forms were found as freely sedimenting molecules. Thus, it is concluded that the replicating forms, when compared with nonreplicating molecules, are preferentially associated with the folded chromosomal structure. Exposure of the folded chromosomal structure to RNase resulted in an unfolding of the chromosome and a concomitant increase in the amount of freely sedimenting replicating and nonreplicating forms of R6K DNA. Analyses of the single-stranded length of RNase-released nascent molecules suggest that they replicate in continuous association with the folded chromsome complex. Nonenzymatic unfolding of the chromosomes by progressively lowering the sodium ion concentration during lysis resulted in a progressive increase in the release of nonreplicating molecules. Replicating molecules wer not released by unfolding the chromosome in this fashion.

Application of R-plasmid DNA's from Escherichia coli minicells in genetic transformation

Components of minicell lyzates of Escherichia coli P678.54 (R1 drd 19) and escherichia coli P678.54 (R6K) were visualized in an electron microscope and used for the transformation of Escherichia coli JC7623. The frequency of the resulting transformants (of the order of 10(-6)%) was not appreciably influenced by the manner of lyzate preparation. The presence of covalently closed circular DNA was detected in two different transformants using radioisotopes, thus demonstrating an autonomous existence of R1 drd 19 or R6K plasmids in tested transformants. This finding corresponds with the results of their genetic analysis.

Transduction of plasmid determinants in Staphylococcus aureus and Escherichia coli

Buoyant density analysis of transducing lysates derived from Staphylococcus aureus and Escherichia coli indicated that phage particles bearing plasmid determinants contain a quantity of DNA equivalent to that found in the lytic particles. Transducing particles that bear plasmid determinants smaller than viral DNA must therefore contain a quantity of DNA in excess of a single plasmid genome. In the E. coli P1vir system, a dependence upon host-mediated recombination for the transduction of small plasmids, but not for large R factors or chromosomal genes, was observed. However, no evidence for the involvement of such functions in the transduction of S. aureus plasmids was obtained. Although the origin of the additional DNA in plasmid transducing particles has not been identified, circumstantial evidence has been presented in the staphylococcal system indicating that transducing particles carrying a small tetracycline plasmid are not formed by the wrapping of multiple copies of this plasmid DNA.

Host-dependent, thermosensitive replication of an R plasmid, pJY5, isolated from Enterobacter cloacae

The thermosensitive replication of an R plasmid, pJY5, isolated from Enterobacter cloacae, was studied. pJY5 consisted of 61 million daltons of covalently closed circular (CCC) deoxyribonucleic acid (DNA) with a buoyant density of 1.714 g/cm3 (55 mol % guanine plus cytosine). In Escherichia coli, this plasmid replicated stringently at 32 degrees C, but ceased its CCC DNA replication after a short incubation at 42 degrees C, resulting in production of R- segregants. The thermosensitive replication of pJY5 was not overcome by the coexistence of non-thermosensitive R plasmids. The plasmid manifested an inhibitory effect on host bacterial cell growth at 42 degrees C, although the effect was less prominent than that of R plasmids belonging to the T-incompatibility group, Rts1, R401, and R402. When the pJY5 plasmid was transferred into E. cloacae, however, no R- segregants were detected at any culture temperature, even 42 degrees C. Alkaline sucrose gradient analysis revealed that a significant amount of pJY5 CCC DNA was synthesized in E. cloacae at the high temperature but not in E. coli. Furthermore, the growth-inhibitory effect of pJY5 on hosts at 42 degrees C was not observed in E. cloacae. On the other hand, Rts1 and R401 were found to be thermosensitive in E. cloacae as well as in E. coli.

Requirement of cyclic adenosine 3',5'-monophosphate for the thermosensitive effects of Rts1 in a cyclic adenosine 3',5'-monophosphate-less mutant of Escherichia coli

Previous publications showed that a covalently closed circular (CCC) Rts1 plasmid deoxyribonucleic acid (DNA) that confers kanamycin resistance upon the host bacteria inhibits host growth at 42 degrees C but not at 32 degrees C. At 42 degrees C, the CCC Rts1 DNA is not formed, and cells without plasmids emerge. To investigate the possible role of cyclic adenosine 3',5'-monophosphate (cAMP) in the action of Rts1 on host bacteria, Rts1 was placed in an Escherichia coli mutant (CA7902) that lacks adenylate cyclase or in E. coli PP47 (a mutant lacking cAMP receptor protein). Rts1 did not exert the thermosensitive effect on these cells, and CCC Rts1 DNA was formed even at 42 degrees C. Upon addition of cAMP to E. coli CA7902(Rts1), cell growth and formation of CCC Rts1 DNA were inhibited at 42 degrees C. The addition of cAMP to E. coli PP47(Rts1) did not cause inhibitory effects on either cell growth or CCC Rts1 DNA formation at 42 degrees C. The inhibitory effect of cAMP on E. coli CA7902(Rts1) is specific to this cyclic nucleotide, and other cyclic nucleotides such as cyclic guanosine 3',5'-monophosphate did not have the effect. For this inhibitory effect, cells have to be preincubated with cAMP; the presence of cAMP at the time of CCC Rts1 DNA formation is not enough for the inhibitory effect. If the cells are preincubated with cAMP, one can remove cAMP during the [(3)H]thymidine pulse and still observe its inhibitory effect on the formation of CCC Rts1 DNA. The presence of chloramphenicol during this preincubation period abolished the inhibitory effect of cAMP. These observations suggest that cAMP is necessary to induce synthesis of a protein that inhibits CCC Rts1 DNA formation and cell growth at 42 degrees C.

Survey of the extrachromosomal gene pool of Streptococcus mutans

Fifty strains of Streptococcus mutans independently isolated from human dental plaque were examined for the presence of covalently closed circular plasmid deoxyribonucleic acid (DNA). Cesium chloride-ethidium bromide centrifugation of [3H]thymidine-labeled, Sarkosyl-lysed cells revealed that 2 of the 50 strains contained plasmid DNA. The plasmid DNA from these strains was characterized by velocity and equilibrium centrifugation and by electron microscopy. The plasmids in these strains were virtually identical in size, with molecular weights of 3.6 X 10(6) and 3.7 X 10(6), Both were present to the extent of approximately 20 molecules per genome equivalent. Interlocked catenated dimeric molecules of each plasmid were readily detected by velocity sedimentation and electron microscopy. These plasmid-containing strains were compared with representative plasmid-free S. mutans strains by using such criteria as bacteriocin production, antibiotic susceptibility, and hemolysis of mammalian erythrocytes. Although no correlation of phenotype to plasmid content could be made, production of bacteriocin-like activity differed significantly between the two plasmid-containing S. mutans isolates. Thus, although the plasmids in these two isolates appeared identical by the criteria of molecular weight, presence of dimers, and copy number, they appeared to be harbored by two distinct S. mutans strains.

Rapid screening for plasmid DNA

A procedure is described for demonstrating plasmid DNA and its molecular weight, based on rate zonal centrifugation of unlabelled DNA in neutral sucrose gradients containing a low concentration of ethidium bromide. Each DNA species is then visualized as a discrete fluorescent band when the centrifuge tube is illuminated with ultra-violet light. Plasmids exist as closed circular and as relaxed circular molecules, which sediment separately, but during preparation of lysates, closed circular molecules are nicked so that each plasmid forms only a single band of relaxed circles within the gradient.

Nonintegrated plasmid-chromosome complexes in Escherichia coli

A number of plasmid systems have been examined for the ability of their covalently closed circular deoxyribonucleic acid (CCC DNA) forms to cosediment in neutral sucrose gradients with the folded chromosomes of their respective hosts. Given that cosedimentation of CCC plasmid and chromosomal DNA represents a bound or complexed state between these replicons, our results can be expressed as follows. (i) All plasmid systems complex, on the average, at least one plasmid per chromosomal equivalent. (ii) Stringently controlled plasmids exist predominantly in the bound state, whereas the opposite is true for plasmids that exist in multiple copies or are under relaxed control of replication. (iii) The degree to which a plasmid population binds to host chromosomes appears to be a function of plasmid genotype and not of plasmid size. (iv) For the colicin E1 plasmid the absolute number of plasmids bound per folded chromosome equivalent does increase as the intracellular plasmid/chromosome ratio increases in cells starved for required amino acids or in cells treated with chloramphenicol; however, the ratio of bound to free plasmids remains constant during plasmid copy number amplification.

Mode of replication of the conjugative R-plasmid RSF1040 in Escherichia coli

Replicating deoxyribonucleic acid (DNA) molecules of plasmid RSF1040, a deletion mutant of the conjugative R plasmid R6K, appear in the electron microscope as partially supercoiled structures with two open circular branches of equal size, although open structures with three branches, two branching points and no supercoiled regions (theta structures) were also found at a lower frequency. The partially supercoiled molecules sediment more rapidly than native covalently closed circular DNA in neutral sucrose gradients and band at a position intermediate between covalently closed circular and open circular DNA in CsClethidium bromide gradients. Electron microscope measurements of the linear EcoRI-treated replicative intermediates indicate that replication can be initiated at two sites (origins) on the plasmid DNA molecule located at about 23% (alpha) and 39% (beta) of the total genome length from an EcoRI end designated arbitrarily as the "left-hand" end of the molecule. The overall replication of RSF1040 is asymmetrically bidirectional. Replication from the alpha origin proceeds first to the "right" to a unique termination site located some 55% of the total genome length from the left-hand end of the molecule. At this point replication proceeds from the alpha origin to the "left" (i.e., opposite to the original direction of replication) until replication of the molecule is completed. Replication also proceeds from the beta origin asymmetrically to the unique terminus site.

Hfr formation by I pilus-determining plasmids in Escherichia coli K-12

R483, an atypical, I pilus-determining plasmid, and also R144, a typical one, were shown to suppress the DnaA phenotype by integration into the Escherichia coli chromosome.

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.

Synergy in the antimicrobial action of penicillin and beta-chloro-D-alanine in vitro

beta-Chloro-d-alanine and penicillin G acted on early and late steps, respectively, in the biosynthesis of the bacterial cell wall. In combination these compounds showed a synergistic effect on the growth of Salmonella typhimurium and of Escherichia coli in vitro.

Indirect selection of bacterial plasmids lacking identifiable phenotypic properties

A procedure is described that uses an indicator plasmid (pSC201) to identify cells in a bacterial population that have been co-transformed with a second plasmid lacking detectable phenotypic properties. Under appropriate conditions of indirect selection, between 50 and 85% of transformants carrying the indicator plasmid also contain the nonselected plasmid. A temperature-sensitive mutation in the replication functions of the indicator plasmid enables its elimination from doubly transformed bacteria. Using this procedure, we have isolated bacteria that carry only the small cryptic plasmid. P15A, of the Escherichia coli strain 15. This genetic element, which contains only 2,300 nucleotide pairs, is thus capable of functioning as a replicon independently of the two larger plasmids normally associated with it in E. coli 15 strains (Ikeda, Inuzuka, and Tomizawa, 1970).

Two replication initiation sites on R-plasmid DNA

Replicating DNA molecules of a deletion mutant of the conjugative R-plasmid R 6 K are cleaved at a single site by the EcoRI restriction endonuclease. Electron microscope examination and measurements of the EcoRI treated replicative intermediate molecules indicate that replication can be initiated at two sites on the plasmid DNA molecule. The two sites are located at about 23 and 39% of total length, respectively, from the EcoRI cleavage site. About 5% of the replicating molecules use both replication initiation sites simultaneously.

Bidirectional replication of plasmid R6K DNA in Escherichia coli; correspondence between origin of replication and position of single-strand break in relaxed complex

Replicating molecules of plasmid R6K DNA have been purified as covalently closed circular DNA forms and analyzed in the electron microscopy after cleavage with the EcoRI restriction endonuclease. It has been determined that in most cases replication proceeds bidirectionally from an origin whose position is indistinguishable from the site of the single-strand break (nick) in the open circular DNA form of the relaxation complex of R6K DNA and protein. Evidence is presented for the existence of a unique replication terminus asymmetrically placed approximately 20% of genome size from the origin. The positions of the replication forks in a majority of the molecules indicate that replication proceeds sequentially from the fixed origin first in one direction to the terminus and then progresses from the origin in the other direction.

Transition of the R factor NR1 and Proteus mirabilis: level of drug resistance of nontransitioned and transitioned cells

When Proteus mirabilis harboring the R factor NR1 is cultured in Penassay broth containing 100 mug of chloramphenicol (CM) per ml, there is an amplification in the number of copies of the r-determinants per cell. Under these conditions, R factors harboring multiple tandem sequences of r-determinants are formed. Autonomous poly-f-determinants consisting of multiple copies of r-determinants are also formed. This phenomenon has been referred to as the "transition". Transitioned cells have considerably higher levels of resistance to CM and streptomycin (SM), but not to tetracycline (TC), than do nontransitioned cells and grow more rapidly in medium containing either CM or SM. There is essentially no difference in growth rates between transitioned and nontransitioned cells in drug-free medium. The higher level of resistance of transitioned cells to SM has made it possible to investigate the mechanism of the transition. Using replica plating, it has been possible to isolate spontaneously occurring transitioned cells from a nontransitioned population which appear to outgrow the nontransitioned cells during growth in medium containing 100 mug of CM per ml. If transiitoned cells are subsequently cultured in drug-free medium, the cells return gradually to the nontransitioned state, which has been referred to as the "back-transition was monitored by examining the level of resisitance of the cells to SM. In both situations the cell populations were found to be heterogeneous, consisting of a mixture of nontransitioned and transitioned cells. Under the conditions of our experiments, the transition appeared to be due to the more rapid growth of a minor fraction of spontaneously occurring transitioned cells which outgrew the remainder of cells in the population. To obtain the transition, the drug resistance gene must reside on the r-determinants component of the R factor. The transition did not take place when the cells were cultured in medium containing high concentrations of TC. This indicates that the TC resistance genes reside on the resistance transfer factor component of the R factor, which is in agreement with physical studies on R factor deoxyribonucleic acid.

Reinitiation of chromosome replication in the presence of chloramphenicol under an integratively suppressed state by R6K

The autonomous replication of an R plasmid, R6K (amp, str) was shown not to be affected by chloramphenicol. It provoked integrative suppression and gave rise to Hfr strains when integrated into the chromosome of a strain of Escherichia coli K-12 with a temperature-sensitive mutation in the gene, dnaA. An Hfr strain designated as Hfr(R6K) no. 1 was thus obtained and characterized. It was not completely stable as shown by a plating efficiency of 0.6 at 42 C relative to that at 30 C. The density labeling and the ultracentrifugation analysis suggested that the deoxyribonucleic acid replication in this Hfr strain did not stop immediately after completion of the round already started before temperature shift-up and the addition of chloramphenicol. These observations are discussed in relation to a possibility that the chromosome replication of this Hfr strain is under the control of the integrated plasmid at a nonpermissive temperature.

Origin of the TEM-beta-lactamase gene found on plasmids

A sequence of deoxyribonucleic acid of 2.7 times 10-6 to 3.3 times 10-6 daltons which includes the TEM beta-lactamase gene is present on the small plasmid RSF 1030 (R-Amp). This same sequence is present on plasmid derivatives that have received a translocation of deoxyribonucleic acid specifying the TEM beta-lactamase and is also present on naturally occurring plasmids of the F1, F11, N, X, O, I, C, and W incompatibility groups that do not specify ampicillin resistance or specify O-type beta-lactamases.

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.

Molecular nature of the deoxyribonucleic acid of a thermosensitive R factor, Rts1

The deoxyribonucleic acid of the thermosensitive R factor, Rts1, has been examined by the technique of sedimentation in alkaline sucrose, electron microscopy, and radiation target size. All these methods yielded a molecular weight of approximately 120 million for Rts1 deoxyribonucleic acid in Escherichia coli. Sedimentation analysis revealed that Rts1 deoxyribonucleic acid in Proteus mirabilis was also 120 million daltons. Rts1 did not segregate into E. coli minicells under the conditions where another smaller non-thermosensitive R factor could.

Co-transduction of plasmids mediating resistance to tetracycline and chloramphenicol in Staphylococcus aureus

Independent plasmids mediating resistance either to tetracycline (Tc) or chloramphenicol (Cm) were transduced successively into Staphylococcus aureus strain 8325. From this doubly resistant donor strain, Tc was co-transduced with a frequency of 40 to 50% when Cm was selected. Co-transduction of Cm was 5 to 10% with Tc selection. Plasmid elimination was infrequent and restricted to the Cm plasmid. A variant, doubly resistant strain gave 100% co-transduction of Tc and Cm and a high rate of joint elimination of both plasmid markers. Co-transduction of the plasmids from recombination-deficient donor strains was much reduced if the plasmids had been introduced separately into the donor strain, but occurred at the normal high rate if they had been introduced jointly. The plasmids were co-transformed at relatively low rates with closed circular deoxyribonucleic acid (DNA) from doubly resistant donors, but not with DNA from a mixed lysate of singly resistant strains. Our evidence favored a hypothesis of recombination-dependent, reversible linkage between the two plasmids as the basis for their co-transduction. Examination of plasmid DNA from the doubly resistant strains by ultracentrifugal and electron microscopic methods did not disclose any physical differences between singly and doubly resistant strains that might account for the observed co-transduction.

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.

Incompatibility reactions of R plasmids isolated from Escherichia coli of animal origin

The incompatibility reactions of a group of 90 R plasmids, isolated from the fecal Escherichia coli of calves, pigs, and chickens, have been determined against reference plasmids of the incompatibility groups F(11), I, N, P, and W. Twenty plasmids belonged to incompatibility groups F(11), I, N, or P. Forty-nine plasmids were compatible with all of the five reference plasmids. Eight plasmids exhibited marked incompatibility with representatives of two or more of the above groups, whereas two strains showed incompatibility reactions consistent with the coexistence of two compatible plasmids. In addition, 19 plasmids remain untyped because they showed intermediate reactions with one or more reference plasmids.

The thermosensitive lesion in the replication of the drug resistance factor, Rts1

The DNA of the thermosensitive R factor, Rts1, has been examined by the technique of sedimentation in alkaline sucrose density gradients. Rts1 DNA was found as closed covalent circles in only a few copies per cell in an Escherichia coli host at the permissive temperature. Rts1 DNA appears to be synthesized at the nonpermissive temperature, but was not found as closed covalent circles. However, circular DNA could be recovered upon shift down to the permissive temperature. The large number of plasmid-negative cells which accumulate after prolonged culture at non-permissive temperature may be due to a strong selective pressure favoring the growth of rare R(-) segregants.