Comparisons of F factors and R factors: existence of independent regulation groups in F factors

Plasmid classifications

Plasmids are universally present in bacteria and play key roles in the dissemination of genes such as antibiotic resistance determinants. Major concepts in Plasmid Biology derive from the efforts to classify plasmids. Here, we review the main plasmid classification systems, starting by phenotype-based methods, such as fertility inhibition and incompatibility, followed by schemes based on a single gene (replicon type and MOB class), and finishing with recently developed approaches that use genetic distances between whole plasmid sequences. A comparison of the latter highlights significant differences between them. We further discuss the need for an operational definition of plasmid species that reveals their biological features, akin to plasmid taxonomic units (PTUs).

Murine Wnt-1 with an internal c-myc tag recombinantly produced in Escherichia coli can induce intracellular signaling of the canonical Wnt pathway in eukaryotic cells

Wnt-1 belongs to the Wnt family of secreted glycoproteins inducing an intracellular signaling pathway involved in cell proliferation, differentiation, and pattern formation. The canonical branch is one of three known branches. This is also valid in vitro, and Wnts can be considered beneficial for culturing primary cells from organs, provided Wnts are available and applicable even with cells of different species. It was shown here that internally c-myc-tagged murine Wnt-1 produced in the heterologous host Escherichia coli was appropriate for inducing intracellular signaling of the canonical Wnt pathway in eukaryotic cells via stabilization of cytosolic beta-catenin. The pioneering injection of the protein into the blastocoels of Xenopus laevis embryos led to axis duplication and suppression of head formation. Applying the recombinant murine Wnt-1 to metanephric mesenchyme activated the tubulogenic program. The signal-inducing activity of the recombinant protein was also positively demonstrated in the TOP-flash reporter assay. Although Wnts were purified recently from the growth media of stably transfected eukaryotic cell lines, the production of active Wnt proteins in pro- or eukaryotic microorganisms reportedly has never been successful. Here soluble production in E. coli and translocation into the oxidizing environment of the periplasm were achieved. The protein was purified using the internal c-myc tag. The effect on the eukaryotic cells implies that activity was retained. Thus, this approach could make recombinant murine Wnt-1 available as a good starting point for other Wnts needed, for example, for maintaining and differentiating stem cells, organ restoration therapy, and tissue engineering.

Knowledge-based assessment of gene expression data from chemiluminescence detection

The first problem in gene expression profiling to be solved is choosing the appropriate gene array, detection procedure, image analysis and data generation depending on the organism of interest, equipment and budget. The next one is how to deduce biologically meaningful data. We assessed gene expression data from chemiluminescent detection and empirically found criteria for the reliable identification of biologically meaningful expression ratios. Current statistical assessments are often applied unreflectedly concerning problems occurring in practice. So interesting results are considered to be irrelevant. This requires a laborious data check. We suggest automation. Our empirically found criteria were transformed into and validated by a knowledge-based system. This system is adaptable to all other methods of expression profiling. We compared the experience-based and new knowledge-based assessment of the expression data from our chemiluminescent and additionally radioactive detection of several experiments with published data to evaluate our entire procedure. With our adaptation of chemiluminescence detection to commercially available Escherichia coli gene arrays we present a useful alternative to common procedures in gene expression monitoring. Moreover, with our consideration of plasmid-harbouring E. coli strains we provide the opportunity to monitor gene expression during processes requiring any plasmids (e.g. recombinant protein expression).

Requirements for strand- and site-specific cleavage within the oriT region of Tn4399, a mobilizing transposon from Bacteroides fragilis

Replicons that contain Tn4399, a conjugal mobilizing transposon isolated from Bacteroides fragilis, can be mobilized in the presence of broad-host-range IncP plasmids RP4 and R751 in Escherichia coli to B. fragilis or E. coli recipients (C. G. Murphy and M. H. Malamy, J. Bacteriol. 175:5814-5823, 1993). To identify the initial DNA processing events involved in Tn4399-mediated mobilization in E. coli, plasmid DNA from pCGM328 (a pUC7 vector that contains the mobilization region of Tn4399) was isolated from donor cells following the release of plasmid DNA from the relaxation complex. Site- and strand-specific cleavage within the oriT region of Tn4399 was detected by denaturing gel electrophoresis and Southern hybridization analysis of this DNA in the presence or absence of IncP plasmids. Mutations in either mocA or mocB, two genes which are encoded by Tn4399 and are required for mobilization, significantly decrease the amount of specifically nicked DNA detected. These results suggest roles for the MocA and MocB gene products in specific processing of Tn4399-containing plasmid DNA prior to mobilization. By isolation of the nicked strand and primer extension of this template, we mapped the precise 5' end of the single-stranded cleavage reaction. The nucleotide position of nicTn4399 is adjacent to two sets of inverted repeats, a genetic arrangement similar to those of previously characterized oriT regions. Two site-directed mutations which remove nicTn4399 (oriT delta 1 and oriT delta 2) cannot be mobilized to recipients when they are present in trans along with functional MocA and MocB proteins and an IncP mobilizing plasmid; they are cis-dominant loss-of-function mutations.

Development of multiple-antibiotic-resistant (Mar) mutants of Pseudomonas aeruginosa after serial exposure to fluoroquinolones

Laboratory-derived fluoroquinolone-resistant mutants were created by serially passaging wild-type Pseudomonas aeruginosa on fluoroquinolone-containing agar to obtain high-level fluoroquinolone resistance (e.g., ciprofloxacin MIC of 1,024 micrograms/ml). With increases of 4- to 32-fold in MICs of fluoroquinolones, these organisms demonstrated (relative to wild-type) normal morphology, resistance to fluoroquinolones only, no change in fluoroquinolone uptake, and no change in lipopolysaccharide profiles or outer membrane protein profiles. Complementation with wild-type Escherichia coli gyrA restored fluoroquinolone susceptibility, suggesting that these were gyrA mutants. After 4- to 32-fold increases in fluoroquinolone MICs (with continued passage on fluoroquinolone-containing agar) isolates demonstrated altered morphology, a multiple-antibiotic-resistant (Mar) phenotype (including cross-resistance to beta-lactams, chloramphenicol, and tetracycline), reduced fluoroquinolone uptake and altered outer membrane proteins (reductions in the 25- and 38-kDa bands as well as several bands in the 43- to 66-kDa region). Complementation with wild-type E. coli gyrA partially reduced the level of fluoroquinolone resistance by approximately 8- to 32-fold, suggesting that these mutants displayed both gyrA and non-gyrA mutations.

Characterization of a "mobilization cassette" in transposon Tn4399 from Bacteroides fragilis

Derivatives of nonconjugal plasmids that carry Tn4399, a transposon isolated from Bacteroides fragilis, can be mobilized for transfer by the broad-host-range IncP plasmids pRK231 or R751 in Escherichia coli. To characterize regions of Tn4399 involved in mobilization, we have isolated and analyzed subcloned fragments of Tn4399 in E. coli, as well as mutations within the element. We have identified a "mobilization cassette" within a 2.8-kb region of Tn4399 which, when cloned into mobilization-deficient plasmids, allows these plasmids to be mobilized in trans by the IncP plasmids pRK231 and R751. The 2.8-kb region has been sequenced, and several open reading frames have been identified. Mutants defective in two genes, designated mocA and mocB, coding for deduced products of 36.4 and 16.4 kDa, respectively, cannot be mobilized by either IncP plasmid; these mutants can be complemented in the presence of the respective wild-type genes in trans. This suggests that the putative MocA and MocB proteins have a role in the mobilization process. The 36.4-kDa MocA protein contains a 14-amino-acid sequence which is closely related to a highly conserved motif within DNA relaxases encoded by a wide variety of conjugal or mobilizable plasmids. Subcloning experiments also lead to the localization of an oriT region within a 199-bp fragment, internal to the mobilization cassette.

Spontaneous quinolone resistance in Serratia marcescens due to a mutation in gyrA

Spontaneous quinolone-resistant mutants of MP050, a quinolone-susceptible clinical strain of Serratia marcescens, were isolated on nutrient agar containing 0.5 microgram of ciprofloxacin per ml. One mutant, designated MP051, was selected for further study. Quinolone MICs for MP051 were 4- to 16-fold higher than those for MP050; nonquinolone MICs were unchanged. The DNA gyrase isolated from MP051 was 24-fold less sensitive to inhibition of supercoiling by ciprofloxacin than the DNA gyrase isolated from MP050 was. Inhibition studies on reconstituted combinations of heterologous gyrase subunits showed that the decreased inhibition was dependent on the A subunit of DNA gyrase from MP051. Further evidence that this decreased inhibition was due to a gyrA mutation was provided by analysis of Escherichia coli gyrA gene expression in S. marcescens heterodiploids containing pNJR3-2, a broad-host-range gyrA gene probe. Quinolone susceptibilities of MP051 heterodiploids containing the wild-type E. coli gyrA gene decreased to those of MP050, while quinolone susceptibilities of MP050 containing the same plasmid were unchanged. These results indicate that spontaneous quinolone resistance in MP051 was due to a mutation in gyrA.

Broad-host-range gyrase A gene probe

The Escherichia coli gyrase A gene was cloned in the broad-host-range cosmid vector pLA2917. The resulting plasmid, pNJR3-2, conferred quinolone susceptibility on a gyrA mutant of E. coli. To analyze the expression of this E. coli gene in Pseudomonas aeruginosa, pNJR3-2 or pLA2917 was mobilized via conjugation into P. aeruginosa PAO2 and several well-characterized quinolone-resistant mutants of this strain. The vector pLA2917 did not significantly affect the quinolone susceptibilities of any of the P. aeruginosa strains. However, pNJR3-2 conferred wild-type quinolone susceptibility on P. aeruginosa cfxA (gyrA) mutants and intermediate quinolone susceptibility on cfxA-cfxB double mutants of P. aeruginosa. The quinolone susceptibility of P. aeruginosa PAO2 gyrA+ was unaffected by pNJR3-2. Also, pNJR3-2 had no significant effect on P. aeruginosa cfxB (permeability) mutants. These results demonstrate that the DNA gyrase A gene from E. coli is expressed in P. aeruginosa and confers dominant susceptibility on gyrA mutants. Thus, pNJR3-2 can be used to detect the quinolone resistance mutations that occur in the gyrase A gene of this organism. pNJR3-2 also appears to discriminate between mutations in gyrA and mutations which alter permeability. This gyrase A probe was used successfully in the analysis of quinolone resistance in clinical isolates of P. aeruginosa.

Factors influencing the accumulation of ciprofloxacin in Pseudomonas aeruginosa

Ciprofloxacin accumulation in Pseudomonas aeruginosa was measured by a bioassay. Drug accumulation in strain PAO2 was compared with that of three spontaneous ciprofloxacin-resistant mutants selected with 0.5 micrograms of ciprofloxacin per ml. PAO4701 cfxA2 contains a mutation in the gyrA gene, PAO4742 cfxB5 may represent a permeability mutant based on pleiotropic drug resistance, and PAO4700 cfxA1 cfxB1 contains both types of mutations. In all strains, drug accumulation was similar, reaching steady state during the first minute of exposure. Drug accumulation was unsaturable over a range of 5 to 80 micrograms/ml, suggesting that ciprofloxacin accumulates by diffusion in P. aeruginosa. Although all four strains accumulated two- to sevenfold more ciprofloxacin in the presence of the inhibitor carbonyl cyanide m-chlorophenylhydrazone, the cfxB mutants accumulated two- to fourfold less drug than either PAO2 or the cfxA2 mutant. Polyacrylamide gel analysis revealed a protein common to cfxB mutants only, while all strains had similar lipopolysaccharide profiles. The results suggest that ciprofloxacin accumulation in P. aeruginosa is a complex phenomenon that may be affected by both an energy-dependent drug efflux process and outer envelope composition.

Characterization of a doubly mutant derivative of the lambda PRM promoter. Effects of mutations on activation of PRM

The mutation, prmE37, located at -14 in the PRM promoter of bacteriophage lambda, reduces PRM function dramatically both in vitro and in vivo. In a search for second-site revertants of prmE37, we isolated a double mutant that exhibits a partially restored Prm+ phenotype. The second-site mutation (at -31) is identical to the mutation prmup-1. The activity of the doubly mutant (pseudo-revertant) promoter, prmE37prmup-1, was investigated in vivo using a PRM-lacZ fusion phage and found to be intermediate between that of prmE37 and wild-type PRM. However, the relative strength of the prmE37prmup-1 promoter was greater than expected following superinfection of a lambda lysogen. Since nalidixic acid was found to preferentially inhibit transcription from the doubly mutant promoter under these conditions, we suggest that DNA supercoiling favors activation of this promoter by repressor. In runoff transcription assays in the absence of repressor, the activity of wild-type PRM and the doubly mutant promoter were the same. However, while addition of repressor significantly stimulated wild-type PRM, it had little or no effect on the activity of the doubly mutant promoter. Values of KB, the equilibrium constant for formation of closed complexes, and kf, the rate constant for isomerization of closed to open complexes, were determined in abortive initiation assays, and the product of kfKB was used as a measure of promoter strength. The results of these assays are in agreement with those obtained in runoff transcription assays. In the absence of repressor, values of kfKB for the doubly mutant promoter and wild-type PRM are the same; however, tau obs, the time required for open complex formation, is significantly greater for the double mutant than for wild-type PRM at all RNA polymerase concentrations used for the abortive initiation analysis. In the presence of repressor, the doubly mutant promoter is stronger than the prmE37 promoter, but much weaker than wild-type PRM. This is due to the fact that kf for the doubly mutant promoter is increased 2.5-fold by repressor, but KB is reduced to the same extent. These two effects counteract each other, so that repressor has no net effect on the strength of the prmE37prmup-1 promoter in vitro. In contrast, repressor increases kf for wild-type PRM eightfold and increases overall promoter strength (KBkf) nearly fivefold. In the presence of repressor, the effects of the two mutations, prmE37 and prmup-1, on kf are independent. This observation is discussed in relation to revised models for open complex formation.

Genetic and physiological characterization of ciprofloxacin resistance in Pseudomonas aeruginosa PAO

Spontaneous ciprofloxacin-resistant mutants of Pseudomonas aeruginosa PAO2 were isolated on ML agar containing 0.5 microgram of ciprofloxacin per ml (2 times the MIC). The mutants were 8- to 64-fold more resistant to ciprofloxacin and showed complete cross resistance to nalidixic acid, ofloxacin, enoxacin, and norfloxacin. Two chromosomal resistance genes, cfxA and cfxB, were mapped between eda-9001 and phe-2 and near pyrB52 distal to proC130, respectively. The cfxB mutation was identical to a nalB mutation and conferred cross resistance to novobiocin, tetracycline, carbenicillin, and chloramphenicol, suggesting that there is an effect on permeability. DNA gyrase A and B subunits were purified from strain PAO2 (wild type), PAO236 nalA2, PAO4704 cfxA2, and PAO4700 cfxA1 cfxB1. Inhibition of gyrase-mediated DNA supercoiling by ciprofloxacin or nalidixic acid was greatly reduced in preparations derived from each of the mutants. Inhibition studies on reconstituted heterologous gyrase subunits showed that decreased inhibition was dependent on the mutant gyrase A subunit. We conclude that ciprofloxacin resistance in P. aeruginosa PAO2 can occur by mutation in the nalB gene or the gene for DNA gyrase A (formerly nalA).

Mutational and in vivo methylation analysis of F-factor PifC protein binding to the pif operator and the region containing the primary origin of mini-F replication

We have used in vivo methods to identify multiple DNA-binding sites for the negatively autoregulated mini-F replication factor PifC. Sequence analysis of pif operator constitutive mutants, isolated as insensitive to repression by PifC, establishes the structure of pifO. This site contains a 17-base-pair (bp) region of dyad symmetry with 7-bp perfect inverted repeats separated by 3 bp. In vivo DNA methylation studies with dimethyl sulfate show that the reactivity of five of six guanine residues in the pifO region is altered in the presence of PifC protein. In addition, there are several sites of PifC-dependent methylation enhancement and protection upstream of pifO within repeated sequences bearing homology to pifO. The significance of the repeated PifC binding sequences and their relationship to the primary origin of mini-F replication (oriV1) are discussed.

Tn4400, a compound transposon isolated from Bacteroides fragilis, functions in Escherichia coli

Transfer factor pBFTM10, isolated from the obligate anaerobic bacterium Bacteroides fragilis, carries a clindamycin resistance determinant which we have suggested is part of a transposable element. DNA homologous to this determinant is found in many Clnr Bacteroides isolates, either in the chromosome or on plasmids. We have now established that Ccr resides on a transposon, Tn4400. In addition to the Ccr determinant that functions under anaerobic conditions in B. fragilis, Tn4400 also carries a determinant for tetracycline resistance (Tcr) which only functions in Escherichia coli under aerobic conditions. The presence of Tn4400 on pBFTM10 does not confer tetracycline resistance on B. fragilis cells containing it. DNA from pBFTM10 was cloned in E. coli, with pDG5 as the cloning vector, to form pGAT500. Using a mobilization assay involving pGAT500 and an F factor derivative, pOX38, we determined that a 5.6-kilobase region of pBFTM10 DNA was capable of mediating replicon fusion and transposition. Most of the mobilization products resulted from inverse transposition reactions, while some were the result of true cointegrate formation. Analysis of the cointegrate molecules showed that three were formed by the action of one of the ends of Tn4400 (IS4400), and one was formed by the action of the whole element (Tn4400). The cointegrate molecule carrying intact copies of Tn4400 at the junction of the two plasmids could resolve to yield an unaltered donor plasmid (pGAT500) and a conjugal plasmid containing a copy of Tn4400 or a copy of one insertion sequence element (pOX38::Tn4400 or pOX38::IS4400). Thus, Tn4400 is a compound transposon containing active insertion sequence elements as directly repeated sequences at its ends.

F factor inhibition of conjugal transfer of broad-host-range plasmid RP4: requirement for the protein product of pif operon regulatory gene pifC

By the use of deletions, point mutations, and gene fusions, we show that the protein product of the F factor pifC gene is responsible for F factor inhibition of plasmid RP4 conjugal transfer. Deletion analysis of pif sequences carried by pSC101-F chimeric plasmids demonstrated that removal of all or part of the pifC coding sequence greatly decreased or abolished the ability of these plasmids to inhibit RP4 transfer. Amber mutations in the pifC gene eliminated inhibition in an Su- host strain but not in and Su+ (supF) host. Plasmids carrying nonpolar pifC mutations did not decrease the efficiency of RP4 transfer when present in trans. Whereas pifC+ plasmids inhibited RP4 transfer, the presence of RP4 in the same cell as F' lac increased F'lac Pif activity approximately 1,000-fold. This effect most likely resulted from the binding of the pifC product to RP4 DNA and concomitant derepression of the F factor pif operon. PifC inhibited trans mobilization of pMS204, a nonconjugative plasmid carrying the RP4 oriT locus, by the RP1 derivative pUB307. pMS204 had no trans effect on pif operon expression, whereas pUB307 increased F'lac Pif expression, as did RP4. Our results suggest that the pifC product inhibits expression of one or more RP4 genes, the products of which are required for conjugal transfer of RP4 and are required in trans for mobilization of nonconjugal RP4 oriT containing plasmids.

Mapping of DNA gyrase cleavage sites in vivo oxolinic acid induced cleavages in plasmid pBR322

We have developed a procedure which permits the mapping of DNA gyrase cleavage sites in vivo. Addition of oxolinic acid, an inhibitor of DNA gyrase, to growing cells of Escherichia coli containing the plasmid pBR322 resulted in double-strand cleavage of DNA, and allowed the isolation of significant quantities of linearized plasmid DNA after lysis of treated cells with sodium dodecyl sulfate. Initially the linear product was purified from agarose gels, cleaved by restriction endonucleases, and then subjected to Southern hybridization analysis using defined DNA probes. A number of distinct cleavage sites, used with varying degrees of efficiency, were identified within pBR322 using this simple procedure. To achieve greater resolution and to improve sensitivity, we then employed an electroblotting procedure to transfer DNA fragments from acrylamide gels onto nylon membranes. This alternative method does not require the isolation of the linearized product before performing the mapping procedure. The improved resolution obtained from acrylamide gels and the superior binding properties of the nylon membranes have allowed us to accurately map 74 distinct oxolinic acid-induced cleavage sites within pBR322. The significance of these findings in light of previously reported studies in vitro, as well as the possible role of such sites during illegitimate recombination, are discussed.

Regulation of the F-factor pif operon: pifO, a site required in cis for autoregulation, titrates the pifC product in trans

F factor pifC, pifA, and pifB gene expression is subject to negative regulation by the product of the pifC locus (J.F. Miller and M. H. Malamy, J. Bacteriol. 156:338-347, 1983). In this paper, we describe the properties of a new regulatory site in the pif region, pifO, which is required in cis for autoregulation of pif gene expression. Spontaneous pifO mutations were isolated that allow expression of a pifC-lacZ protein fusion in the presence of pifC product in trans. Recombination of these pifO mutations onto F'lac results in increased pifA and pifB activity. Thus, a single regulatory element, pifO, regulates pifC, pifA, and pifB expression in cis. The presence of multiple copies of a fragment from the pif region carrying wild-type pifO sequences (F coordinates 42.9 to 42.43 kilobases) in trans to F'lac results in an increase in pifA and pifB activity as measured by inhibition of T7 plating. When the pifO mutations are recombined onto a plasmid carrying pifO, the resulting recombinants are greatly decreased in the ability to increase F'lac pif expression. These results suggest that increased F'lac pifA and pifB expression caused by pifO sequences in trans is a consequence of titration of pifC product and derepression of the pif operon.

Identification of the pifC gene and its role in negative control of F factor pif gene expression

The pif region of the F factor includes two genes, pifA and pifB, that lead to abortive T7 infection. We have identified a new gene in this region, pifC, by constructing an in vitro fusion of pif DNA at 41.6 kilobases on the F factor physical map to the lacZ gene. A PifC-LacZ fusion protein of 149,000 daltons has been identified by immunoprecipitation and polyacrylamide gel electrophoresis. This allows us to assign the N terminus of pifC to 42.5 kilobases on the F map. Using fusions of pifC, pifA, and pifB to lacZ, we have studied the regulation of pif gene expression and have shown that the product of pifC negatively controls its own expression and that of pifA and pifB.

Cloning of the pif region of the F sex factor and identification of a pif protein product

This paper reports a detailed investigation of the pif region of the F factor responsible for inhibition of development of T7 and related "female-specific" phages. We have mapped a series of pif::Tn5 insertions to a region between 39.6 and 42.8 kilobases on the physical map of F. All pif::Tn5 insertions plated T7 at full efficiency; most were clustered in a 1.8-kilobase interval on both sides of the EcoRI site located at F coordinate 40.3 kilobases. A 5.2-kilobase Pst-I fragment with F coordinates 38.9 to 44.1 has been cloned into a pSC101 vector to create the Pif+ plasmid pGS103. A series of Pif- deletion mutants and nonsense mutants were isolated from pGS103. Using minicells carrying pGS103 or its derivatives, we have identified a 70,000-dalton pif protein.

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.

Studies on plasmid replication. IV. Complementation of replication-defective mutants by an incompatibility-deficient plasmid

This paper described a complementation test system for replication-defective S. aureus pencillinase plasmids in which the incompatibility barrier has been overcome by the isolation of an incompatibility-defective (Inc-) plasmid. This plasmid appears to be stably and irreversibly integrated into the host chromosome as attempts to restore it to its original independent state have been unsuccessful. The Inc- plasmid was able to complement the thermosensitive replication defects of Seg- plasmids belonging to the same original incompatibility class but was unable to complement onels belonging to a different incompatibility class. Positive and negative phenotypic complementation tests were confirmed at the molecular level by isotopec labeling of plasmid-specific DNA molecules.

Evidence for the presence of nontranslated T7 late mRNA in infected F'(PIF+) episome-containing cells

The inability of T7 to develop in cells of Escherichia coli containing F(+) or substituted F' episomes is a result of the failure to synthesize late proteins; no in vivo translation of mRNA species synthesized by the T7 RNA polymerase occurs. Further experiments have been performed to measure the amount of late mRNA in T7-infected F'(PIF(+)) cells. (We have designated the property of phage inhibition of F factors as PIF; the wild-type episome is therefore F'[PIF(+)].) T7 late proteins were synthesized in vitro by using a system programed with RNA extracted from T7-infected F(-) and F'(PIF(+)) cells. The T7 lysozyme, product of gene 3.5, and the gene 10 head protein were assayed. The following results were obtained: (i) mRNA capable of supporting in vitro synthesis of lysozyme and the gene 10 head protein is present in T7-infected F'(PIF(+)) cells; (ii) lysozyme mRNA extracted from T7-infected F'(PIF(+)) cells is present at 70 to 75% of the level found in T7-infected F(-) cells; (iii) gene 10 mRNA is present at 35 to 78% of the level found in T7-infected F(-) cells. No in vivo synthesis of either lysozyme or gene 10 protein can be detected in T7-infected F'(PIF(+)) cells although normal synthesis of these proteins occurs in F(-) cells. These findings confirm that the block in T7 development in F'(PIF(+)) cells results from the failure to translate late classes of T7 RNA.

Molecular studies on entry exclusion in Escherichia coli minicells

Minicells produced by abnormal cell division in a strain of Escherichia coli (K-12) have been employed here to investigate the phenomenon of "entry exclusion." When purified minicells from strains containing F' or R factors, or both, are mated with radioactive thymidine-labeled Hfr or R(+) donors, the recipient minicells can be conveniently separated from normal-sized donors following mating, and the products of conjugation can be analyzed in the absence of donors and of further growth of the recipients. Transmissible plasmids or episomes are transferred less efficiently to purified minicells derived from strains carrying similar or related elements than to strains without them. Measurement of deoxyribonucleic acid (DNA) degradation and determination of weight-average molecular weights following transfer indicate that degradation of transferred DNA or transfer of smaller pieces cannot account for the comparative reduction in transfer to entry-excluding recipients. Therefore, we conclude that entry exclusion operates to prevent the physical entry of DNA into recipients expressing the exclusion phenotype. The R-produced repressor (product of the drd(+) gene), which represses fertility (i.e., ability to act as donor), reduces exclusion mediated by R or F factor, or both, in matings between strains carrying homologous elements. Furthermore, the data suggest that the presence of the F pilus or F-like R pilus on recipient cells ensures maximum expression of the exclusion phenotype but is not essential for its expression. In contrast to previous suggestions, we found no evidence for a reduction of entry exclusion attributable to the DNA temperature-sensitive chromosomal mutation dnaB(TS).

Naturally occurring R factor, derepressed for pilus synthesis, belonging to the same compatibility group as the sex factor F of Escherichia coli K-12

A naturally occurring R factor with constitutive pilus synthesis is described which resembles the sex factor F in compatibility and in restricting coliphage T7. Unlike F, it is not cured during growth with acridine orange. Results suggest that the R factor produces repressor of pilus synthesis, to which the operator is insensitive (i(+)o(c)). In this respect it differs from both the F factor (i(-)o(+)) and wild-type F-like R factors (i(+)o(+)).