Conjugal transfer of bacterial chromosomes mediated by the RK2 plasmid transfer origin cloned into transposon Tn5

Benefits of a Recombination-Proficient Escherichia coli System for Adaptive Laboratory Evolution

Adaptive laboratory evolution typically involves the propagation of organisms asexually to select for mutants with the desired phenotypes. However, asexual evolution is prone to competition among beneficial mutations (clonal interference) and the accumulation of hitchhiking and neutral mutations. The benefits of horizontal gene transfer toward overcoming these known disadvantages of asexual evolution were characterized in a strain of Escherichia coli engineered for superior sexual recombination (genderless). Specifically, we experimentally validated the capacity of the genderless strain to reduce the mutational load and recombine beneficial mutations. We also confirmed that inclusion of multiple origins of transfer influences both the frequency of genetic exchange throughout the chromosome and the linkage of donor DNA. We built a simple kinetic model to estimate recombination frequency as a function of transfer size and relative genotype enrichment in batch transfers; the model output correlated well with the experimental data. Our results provide strong support for the advantages of utilizing the genderless strain over its asexual counterpart during adaptive laboratory evolution for generating beneficial mutants with reduced mutational load.

IMPORTANCE

Over 80 years ago Fisher and Muller began a debate on the origins of sexual recombination. Although many aspects of sexual recombination have been examined at length, experimental evidence behind the behaviors of recombination in many systems and the means to harness it remain elusive. In this study, we sought to experimentally validate some advantages of recombination in typically asexual Escherichia coli and determine if a sexual strain of E. coli can become an effective tool for strain development.

Adaptation of the yeast URA3 selection system to gram-negative bacteria and generation of a {delta}betCDE Pseudomonas putida strain

A general procedure for efficient generation of gene knockouts in gram-negative bacteria by the adaptation of the Saccharomyces cerevisiae URA3 selection system is described. A Pseudomonas putida strain lacking the URA3 homolog pyrF (encoding orotidine-5'-phosphate decarboxylase) was constructed, allowing the use of a plasmid-borne copy of the gene as the target of selection. The delivery vector pTEC contains the pyrF gene and promoter, a conditional origin of replication (oriR6K), an origin of transfer (mobRK2), and an antibiotic selection marker flanked by multiple sites for cloning appropriate DNA segments. The versatility of pyrF as a selection system, allowing both positive and negative selection of the marker, and the robustness of the selection, where pyrF is associated with uracil prototrophy and fluoroorotic acid sensitivity, make this setup a powerful tool for efficient homologous gene replacement in gram-negative bacteria. The system has been instrumental for complete deletion of the P. putida choline-O-sulfate utilization operon betCDE, a mutant which could not be produced by any of the other genetic strategies available.

Modified RP4 and Tn5-Mob derivatives for facilitated manipulation of large plasmids in Gram-negative bacteria

We have constructed a set of RP4 (NmS/TcS) and Tn5-Mob derivatives which have applications in experiments involving mobilization of replicons in many Gram-negative organisms. The different selection markers of the RP4 and Tn5-Mob derivatives include streptomycin, chloramphenicol, gentamicin, and spectinomycin resistance as well as mercury resistance, and a constitutively expressed lacZ gene. This choice of markers allows the use of these derivatives in bacteria which are naturally resistant to many antibiotics, and in strains which contain pre-existing resistance plasmids, transposons, or antibiotic cassette insertions. In addition, a RP4 derivative carrying the sacB gene of Bacillus subtilis was constructed. This allows the selection for the loss of RP4 after it has been used to mobilize other plasmids. A Tn5-Mob-sacB derivative with a new marker (Gm) was also developed, as were vectors which take advantage of the sacB gene to facilitate replacement of existing Tn5 inserts with other Tn5 derivatives. As an example of the use of these tools, three Rhizobium leguminosarum bv. viciae VF39 plasmids which have been shown to be involved in symbiosis were differentially tagged and mobilized (individually and in various combinations) to the plasmid-free Agrobacterium tumefaciens strain UBAPF2. None of the resultant Agrobacterium strains was able to fix nitrogen in symbiosis with peas.

oriT-directed cloning of defined large regions from bacterial genomes: identification of the Sinorhizobium meliloti pExo megaplasmid replicator region

We have developed a procedure to directly clone large fragments from the genome of the soil bacterium Sinorhizobium meliloti. Specific regions to be cloned are first flanked by parallel copies of an origin of transfer (oriT) together with a plasmid replication origin capable of replicating large clones in Escherichia coli but not in the target organism. Supplying transfer genes in trans specifically transfers the oriT-flanked region, and in this process, site-specific recombination at the oriT sites results in a plasmid carrying the flanked region of interest that can replicate in E. coli from the inserted origin of replication (in this case, the F origin carried on a BAC cloning vector). We have used this procedure with the oriT of the plasmid RK2 to clone contiguous fragments of 50, 60, 115, 140, 240, and 200 kb from the S. meliloti pExo megaplasmid. Analysis of the 60-kb fragment allowed us to identify a 9-kb region capable of autonomous replication in the bacterium Agrobacterium tumefaciens. The nucleotide sequence of this fragment revealed a replicator region including homologs of the repA, repB, and repC genes from other Rhizobiaceae, which encode proteins involved in replication and segregation of plasmids in many organisms.

Conjugative transfer of tet(S) between strains of Enterococcus faecalis is associated with the exchange of large fragments of chromosomal DNA

The tetracycline resistance determinant tet(S) was first detected in antibiotic multiresistant Listeria monocytogenes BM4210 and subsequently in strains of Enterococcus faecalis. Transfer of tet(S) from clinical isolate E. faecalis BM4242 to E. faecalis strains JH2-2 and OG1RF was found to require the presence in the donor strain of the 55 kb conjugative plasmid pIP825. Comparison of restriction endonuclease generated maps of the donor, the two recipients, and of four transconjugants indicated that transfer of tet(S) (i) was from chromosome to chromosome, (ii) resulted in the acquisition of an approximately 40 kb element in the same chromosomal region and (iii) was associated with the exchange of large chromosomal fragments. Similar observations were made following conjugal transfer of tet(S) from four other E. faecalis clinical isolates.

Central regulatory role for the RpoS sigma factor in expression of Salmonella dublin plasmid virulence genes

The plasmid virulence genes spvABCD of Salmonella spp. are regulated by SpvR and the stationary-phase sigma factor RpoS. The transcription of spv genes is induced during the post-exponential phase of bacterial growth in vitro. We sought to investigate the relationship between growth phase and RpoS in spv regulation. rpoS insertion mutations were constructed in S. dublin Lane and plasmid-cured LD842 strains, and the mutants were found to be attenuated for virulence and deficient in spv gene expression. We utilized the plasmid pBAD::rpoS to express rpoS independent of the growth phase under the control of the arabinose-inducible araBAD promoter. SpvA expression was induced within 2 h after the addition of 0.1% arabinose, even when bacteria were actively growing. This suggested that the level of RpoS, instead of the growth phase itself, controls induction of the spv genes. However, RpoS did not activate transcription of spvA in the absence of SpvR protein. Using a constitutive tet promoter to express spvR, we found that the spvA gene can be partially expressed in the rpoS mutant, suggesting that RpoS is required for SpvR synthesis. We confirmed that spvR is poorly expressed in the absence of RpoS. With an intact rpoS gene, spvR expression is not dependent on an intact spvR gene but is enhanced by spvR supplied in trans. We propose a model for Salmonella spv gene regulation in which both RpoS and SpvR are required for maximal expression at the spvR and spvA promoters.

Hfr mapping of mutations in Bordetella pertussis that define a genetic locus involved in virulence gene regulation

We report the development of techniques for the genetic mapping of point mutations in the bacterial pathogen Bordetella pertussis. A plasmid vector which is self-transmissible by conjugation and which, by insertion into the B. pertussis chromosome, can mobilize chromosomal sequences during conjugation with a recipient B. pertussis bacterium has been constructed. This vector is used in conjunction with a set of strains containing kanamycin resistance gene insertions at defined physical locations in the B. pertussis genome. In crosses between these donor strains and a mutant recipient strain, transfer of a chromosomal segment flanking the kanamycin resistance gene insertion is selected for, and the percentage of exconjugants which reacquire the wild-type trait is scored. In this way the linkage of the mutant allele to these markers, and thus the approximate chromosomal position of the mutant allele, is determined. We have used this genetic system to map a newly described locus in B. pertussis involved in the regulation of the virulence genes ptx (pertussis toxin) and cya (adenylate cyclase toxin).

Shuttle mutagenesis of Legionella pneumophila: identification of a gene associated with host cell cytopathicity

We performed shuttle mutagenesis of Legionella pneumophila. Mutants were screened for reduced cellular infectivity. Approximately 10% of the mutants had decreased cytopathicity. The DNA sequence of one locus was determined; the inferred amino acid sequence revealed homology with transport proteins including Escherichia coli TolC, Bordetella pertussis CyaE, and Alcaligenes eutrophus CzcC and CnrC.

Mutagenesis of Legionella pneumophila using Tn903 dlllacZ: identification of a growth-phase-regulated pigmentation gene

Study of the molecular basis for Legionella pneumophila pathogenicity would be facilitated with an efficient mutagen that can not only mark genomic mutations, but can also be used to reflect gene expression during macrophage infection. A derivative of Tn903, Tn903dlllacZ, is shown to transpose with high efficiency in L. pneumophila. Tn903dlllacZ encodes resistance to kanamycin (KmR) and carries a 5' truncated 'lacZ gene that can form translational fusions to L. pneumophila genes upon transposition. The cis-acting Tn903 transposase is supplied outside Tn903dlllacZ, and hence chromosomally integrated copies are stable. KmR LacZ+ insertion mutants of L. pneumophila were isolated and shown by DNA hybridization to carry a single Tn903dlllacZ inserted within their chromosomes at various locations. One particular KmR LacZ+ mutant, AB1156, does not produce the brown pigment (Pig-) characteristic of Legionella species. Tn903dlllacZ is responsible for this phenotype since reintroduction of the transposon-linked mutation into a wild-type background results in a Pig- phenotype. L. pneumophila pigment production is normally observed in stationary-phase growth of cells in culture, and beta-galactosidase activity measured from the pig::lacZ fusion increased during the logarithmic-phase growth and peaked at the onset of stationary phase. Interestingly, pig::lacZ expression also increased during macrophage infection. The pigment itself, however, does not appear to be required for L. pneumophila to grow within or kill host macrophages.

Characterization of the Campylobacter fetus sapA promoter: evidence that the sapA promoter is deleted in spontaneous mutant strains

Wild-type Campylobacter fetus cells possess S-layer proteins (S+ phenotype), whereas after laboratory passage, spontaneous stable mutants that do not express these proteins (S- phenotype) arise. To determine the molecular mechanisms by which C. fetus changes to the S- phenotype, we studied wild-type strain 23D, from which the sapA gene encoding the 97-kDa S-layer protein has been cloned, and strain 23B, a spontaneous S- mutant. We compared these strains with another pair of strains, LP (S+) and HP (S-). Southern analysis with the cloned sapA gene as a probe indicated that both pairs of strains have multiple sapA homologs. Using gene disruption and replacement techniques, we constructed an isogenic strain of 23D that differed only in sapA expression (strain 23D:401:1). A 6.0-kb HindIII fragment from 23D:401:1 containing 3.4 kb of sapA upstream region then was cloned into pBluescript to produce pBG101. Nucleotide sequence analysis of sapA upstream region revealed a consensus promoter at -121 bp from the translational start site. Primer extension analysis placed a single in vivo transcription initiation site at the -114-bp position of sapA. A DNA probe derived from the sapA promoter region hybridized to a 5.5-kb HindIII fragment of chromosomal DNA from strain 23D but not to DNA from strain 23B. Northern RNA blot analysis showed no sapA mRNA in strain 23B. These data indicate that the lack of S-layer protein expression in spontaneous mutant strains is caused by the deletion of promoter sequences.

Intergeneric conjugation in Thiobacillus versutus

In plate matings with Escherichia coli HB101/pUW965::Tn5 (KmR) Thiobacillus versutus reacted as an efficient recipient, producing 10(-2) to 10(-3) kanamycin resistant (KmR) T. versutus exconjugants per donor cell. Analysis of agarose gels of plasmid DNA extracted from the exconjugants confirmed that the suicide vector pUW964 did not persist in the recipient, implying that the kanamycin resistance of the exconjugants is based on effective transposition of Tn5 in T. versutus as well as function of the E. coli kanamycin gene. Transfer was equally efficient when a nalidixate-resistant T. versutus mutant was used as recipient. Hybridization evidence for the presence of Tn5 was consistently negative. The significance of this anomalous result is discussed.

Legionella pneumophila mip gene potentiates intracellular infection of protozoa and human macrophages

Legionella pneumophila is an intracellular parasite of freshwater protozoa and human macrophages. Recent studies determined that the macrophage infectivity potentiator (Mip) surface protein, a prokaryotic homolog of the FK506-binding proteins, is required for optimal infection of macrophages. To determine whether Mip is also involved in L. pneumophila infection of protozoa, we examined the ability of a strain lacking Mip to parasitize Hartmannella amoebae and Tetrahymena ciliates. After 3 days of incubation, approximately 1000-fold fewer bacteria were recovered from protozoan cocultures infected with the Mip- strain than from those cocultures infected with an isogenic Mip+ strain. The mip mutant was, however, not impaired in its ability to bind to amoebae cell surfaces, indicating that Mip is involved in bacterial resistance to intracellular killing and/or intracellular multiplication. These data suggest that L. pneumophila employs similar genes and mechanisms to infect human cells and protozoa. Furthermore, they support the hypothesis that the ability of L. pneumophila to parasitize macrophages and hence to cause human disease is a consequence of its prior adaptation to intracellular growth within protozoa.

Cloning and characterization of a gene required for the secretion of extracellular enzymes across the outer membrane by Xanthomonas campestris pv. campestris

Nonpathogenic mutants of Xanthomonas campestris pv. campestris, generated from transposon mutagenesis, accumulated extracellular polygalacturonate lyase, alpha-amylase, and endoglucanase in the periplasm. The transposon Tn5 was introduced by a mobilizable, suicidal plasmid, pSUP2021 or pEYDG1. Genomic banks of wild-type X. campestris pv. campestris, constructed on the broad-host-range, mobilizable cosmid pLAFR1 or pLAFR3, were conjugated with one of the mutants, designated XC1708. Recombinant plasmids isolated by their ability to complement XC1708 can be classified into two categories. One, represented by pLASC3, can complement some mutants, whereas the other, represented by a single plasmid, pLAHH2, can complement all of the other mutants. Restriction mapping showed that the two recombinant plasmids shared an EcoRI fragment of 8.9 kb. Results from subcloning, deletion mapping, and mini-Mu insertional mutation of the 8.9-kb EcoRI fragment suggested that a 4.2-kb fragment was sufficient to complement the mutant XC1708. Sequence analysis of this 4.2-kb fragment revealed three consecutive open reading frames (ORFs), ORF1, ORF2, and ORF3. Hybridization experiments showed that Tn5 in the genome of XC1708 and other mutants complemented by pLASC3 was located in ORF3, which could code for a protein of 83.5 kDa. A signal peptidase II processing site was identified at the N terminus of the predicted amino acid sequence. Sequence homology of 51% was observed between the amino acid sequences predicted from ORF3 and the pulD gene of Klebsiella species.

A directional, high-frequency chromosomal mobilization system for genetic mapping of Rhizobium meliloti

A system for mapping of the Rhizobium meliloti chromosome that utilizes transposon Tn5-Mob, which carries the mobilization site of IncP plasmid RP4 (R. Simon, Mol. Gen. Genet. 196:413-420, 1984), was developed. Insertions of Tn5-Mob that were located at particular sites on the R. meliloti chromosome were isolated and served as origins of high-frequency chromosomal transfer when IncP tra functions were provided in trans. This approach is, in principle, applicable to any gram-negative bacterium in which Tn5 can transpose and into which IncP plasmids can conjugate.

Characterization of three proteins expressed from the virulence region of plasmid pSDL2 in Salmonella dublin

Infection of both cattle and humans with Salmonella dublin can result in septicemia and death. Like many nontyphoid Salmonella species that cause disease, S. dublin contains a cryptic plasmid (pSDL2) that is required for the full expression of virulence. Transposon mutagenesis of pSDL2 defined a 4.1-kb EcoRI region that is necessary for the development of a systemic infection in BALB/c mice. This EcoRI fragment was cloned into an expression vector (pEL11), and three proteins produced from this region with apparent molecular weights of 30,500, 76,000, and 27,000 were identified. Because bacterial proteins that play a role in virulence are often associated with the outer membrane, we were interested in establishing whether the proteins expressed from the EcoRI fragment are located in the membrane. Transposon mutagenesis of pEL11 with TnphoA defined the order of the genes along the fragment and suggested that the proteins may be exported out of the cytoplasm. Sucrose gradient cell fractionation was done to identify the cellular location of each of the three proteins. The 30-kDa protein was identified in the outer membrane fraction, and the 76-kDa protein was located in the cytosolic fraction. The 27-kDa protein was identified in both the cytosolic and the outer membrane fractions. The outer membrane contained less than 10% of the activity of enzymes known to be located in the cytoplasm, periplasm, and inner membrane. Sequence data of the 4.1-kb EcoRI region revealed that both the 30- and the 27-kDa proteins lack a typical signal sequence for export out of the cytoplasm (M. Krause, C. Roudier, J. Fierer, J. Harwood, and D. G. Guiney, Mol. Microbiol. 5:307, 1991). The outer membrane location of these proteins suggests that they may be exported out of the cytoplasm by an unusual mechanism.

Genetic map of Rhizobium meliloti megaplasmid pRmeSU47b

A circular linkage map of the Rhizobium meliloti megaplasmid pRmeSU47b was constructed. The map consists of transposon insertions carrying alternating antibiotic resistance markers linked by phi M12 transduction. Data from conjugation experiments utilizing donor strains carrying Tn5-oriT insertions in the megaplasmid supported the proposed genetic map. In addition, the positions of previously identified Fix, exopolysaccharide synthetic, thiamine synthetic, and C4-dicarboxylate transport loci on the megaplasmid map were determined. By converting cotransduction frequencies to physical distance, we calculated the replicon to be 1,600 kilobases in size, which compares favorably with previous physical estimates.

Correlation between the presence of sequences homologous to the vir region of Salmonella dublin plasmid pSDL2 and the virulence of twenty-two Salmonella serotypes in mice

Large plasmids encoding important virulence properties have been found in several Salmonella serotypes. We have studied the relationship between the presence of a highly conserved 4-kilobase (kb) EcoRI fragment from the plasmid virulence region and pathogenicity for mice of 53 isolates representing 22 serotypes of Salmonella. Only strains possessing the homologous 4-kb region were virulent for mice. In addition, we transferred the virulence plasmid from S. dublin into nine different serotypes, including S. typhi and S. paratyphi A, that lack a native virulence plasmid. Only S. heidelberg and S. newport were rendered mouse virulent by the introduction of the S. dublin plasmid. This study demonstrates that plasmid-mediated virulence sequences are required for Salmonella virulence in mice, but many strains, including the agents of human typhoid fever, also lack chromosomal genes necessary to produce lethal systemic disease in mice. Since all the major Salmonella strains that are host-adapted to animals carry virulence plasmids, it appears that these plasmids are important in mediating systemic infection in animals and may contribute to septicemic, nontyphoid salmonellosis in humans.

Analysis of protein localization by use of gene fusions with complementary properties

This report describes a new transposon designed to facilitate the combined use of beta-galactosidase and alkaline phosphatase gene fusions in the analysis of protein localization. The transposon, called TnlacZ, is a Tn5 derivative that permits the generation of gene fusions encoding hybrid proteins carrying beta-galactosidase at their C termini. In tests with plasmids, TnlacZ insertions that led to high cellular beta-galactosidase activity were restricted to sequences encoding either cytoplasmic proteins or cytoplasmic segments of a membrane protein. The fusion characteristics of TnlacZ are thus complementary to those of TnphoA, a transposon able to generate alkaline phosphatase fusions whose high-activity insertion sites generally correspond to periplasmic sequences. The structure of TnlacZ allows the conversion of a TnlacZ fusion into the corresponding TnphoA fusion (and vice versa) through recombination or in vitro manipulation in a process called fusion switching. Fusion switching was used to generate the following two types of fusions with unusual properties: a low-specific-activity beta-galactosidase-alkaline phosphatase gene fusion and two toxic periplasmic-domain serine chemoreceptor-beta-galactosidase gene fusions. The generation of both beta-galactosidase and alkaline phosphatase fusions at exactly the same site in a protein permits a comparison of the two enzyme activities in evaluating the subcellular location of the site, such as in studies of membrane protein topology. In addition, fusion switching makes it possible to generate gene fusions whose properties should facilitate the isolation of mutants defective in the export or membrane anchoring of different cell envelope proteins.

New derivatives of transposon Tn5 suitable for mobilization of replicons, generation of operon fusions and induction of genes in gram-negative bacteria

Three types of new variants of the broad-host-range transposon Tn5 are described. (i) Tn5-mob derivatives with the new selective resistance (R) markers GmR, SpR and TcR facilitate the efficient mobilization of replicons within a wide range of Gram-negative bacteria. (ii) Promoter probe transposons carry the promoterless reporter genes lacZ, nptII, or luc, and NmR, GmR or TcR as selective markers. These transposons can be used to generate transcriptional fusions upon insertion, thus facilitating accurate determinations of gene expression. (iii) Tn5-P-out derivatives carry the npt- or tac-promoter reading out from the transposon, and TcR, NmR or GmR genes. These variants allow the constitutive expression of downstream genes. The new Tn5 variants are available on mobilizable Escherichia coli vectors suitable as suicidal carriers for transposon mutagenesis of non-E. coli recipients and some on a phage lambda mutant to be used for transposon mutagenesis in E. coli.

A second exopolysaccharide of Rhizobium meliloti strain SU47 that can function in root nodule invasion

Rhizobium meliloti strain SU47 produces the calcofluor-binding exopolysaccharide, succinoglycan, that is required for alfalfa root nodule invasion. Strains derived from R. meliloti SU47 secreted an acidic exopolysaccharide, EPSb, that replaced succinoglycan in nodule invasion. EPSb, which has not formerly been identified among the Rhizobiaceae, consisted of the repeating unit 4,6-O-(1-carboxyethylidene)-alpha-D-Galp1----3(X-O-Ac)-beta-D-G lcp1----3. EPSb synthesis occurred either in strains containing a mutation in a locus designated mucR or in strains with a recombinant cosmid pMuc. mucR mapped slightly counterclockwise from pyr49 on the chromosome, while pMuc contained genes mapping to the megaplasmid pRmeSU47b. In exoA, -F, and -H mutants, which are deficient in normal succinoglycan secretion and nodule invasion, a transposon Tn5 insertion in mucR or the presence of pMuc resulted in EPSb secretion and a restoration of nodule invasion on Medicago sativa and Melilotus alba. Mutants in exoB and exoC were incapable of succinoglycan and EPSb secretion as well as nodule invasion. A mutant that secreted succinoglycan but was incapable of EPSb secretion invaded nodules normally.

A Legionella pneumophila gene encoding a species-specific surface protein potentiates initiation of intracellular infection

To investigate the pathogenesis of Legionnaires disease at a molecular level, we mutated by directed allelic exchange a gene encoding a Legionella pneumophila-specific 24,000-dalton (Da) surface protein. Southern hybridization and immunoblot analyses demonstrated that the predicted DNA rearrangement occurred in L. pneumophila with a specific loss of 24-kDa antigen expression. Compared with its isogenic parent, the mutant was significantly impaired in its ability to infect transformed U937 cells, a human macrophagelike cell line; i.e., the bacterial inoculum of the mutant strain that was required to initiate infection of the macrophage monolayer was ca. 80-fold greater than that of the isogenic parent strain. The mutant strain regained full infectivity on reintroduction of a cloned 24-kDa protein gene, indicating that the reduced infectivity was due specifically to the mutation in that gene. Compared with the parent strain, the mutant strain was recovered at titers that were ca. 10-fold lower shortly after infection, but it exhibited a similar intracellular growth rate over the next 40 h, indicating that the mutant was defective in its ability to initiate macrophage infection rather than in its ability to replicate intracellularly. When opsonized, the mutant strain was still significantly less infectious than the parent strain, despite equivalent macrophage association, suggesting that the mutant was not merely missing a ligand for macrophage attachment. The mutant also exhibited reduced infectivity in explanted human alveolar macrophages, demonstrating the relevance of the U937 cell model for analyzing this mutant phenotype. These results represent the first identification of a cloned L. pneumophila gene that is necessary for optimal intracellular infection; we designate this gene mip, for macrophage infectivity potentiator.

Location of the relaxation complex nick site within the minimal origin of transfer region of RK2

Transfer of plasmid DNA during bacterial conjugation begins at a specific site: the origin of transfer (oriT). The oriT region of the broad host range plasmid RK2 is located on a 250 bp fragment. Deletions involving either end of this region reduce transfer function, indicating that an extended sequence is required for optimal oriT activity. The single-strand nick induced by the RK2 DNA-protein relaxation complex is located adjacent to the 19 bp inverted repeat within the minimal oriT sequence. These results provide strong evidence that the plasmid relaxation event induced in vitro represents the nicking reaction that initiates DNA transfer at oriT during conjugation.

Construction of transposons carrying the transfer functions of RP4

The transfer genes and origin of transfer of the wide host range plasmid RP4 have been cloned into the transposons Tn1 and Tn5. The newly constructed transposons can be used to mutagenize bacterial plasmids or the chromosome in species such as Escherichia coli or Rhizobium. It is then possible to mobilize the plasmid or chromosome using the transfer functions provided in cis by the transposon. These constructs may aid chromosome mapping in many gram-negative species by allowing the wider use of the RP4 conjugal transfer system combined with the potential ability to select the site of insertion and thus the site of the origin of transfer.

Transfer and maintenance of small, mobilizable plasmids with ColE1 replication origins in Legionella pneumophila

With the mutagenesis of specific, virulence-associated genes of Legionella pneumophila as the eventual goal, methods for gene transfer to these bacteria were developed. Following the observations of others that conjugative, broad-host-range plasmids could be transferred from Escherichia coli to L. pneumophila at low frequency, we constructed a small mobilizable vector, pTLP1, which carries oriV from pBR322, oriT from pRK2, Kmr from Tn5, and an L. pneumophila-derived fragment to permit chromosomal integration. In triparental matings including an E. coli with a conjugative (Tra+) helper plasmid, kanamycin-resistance was transferred from E. coli to L. pneumophila. Southern hybridization of L. pneumophila transconjugants showed that pTLP1 was replicated autonomously. Additional matings of plasmids having deletions or substitutions of pTLP1 sequences confirmed that replication in L. pneumophila requires oriV only. pTLP1 was maintained in L. pneumophila with passage on medium containing kanamycin but was rapidly lost after passage on nonselective medium. This plasmid instability in L. pneumophila is most likely due to rapid generation of plasmid-free segregants because of plasmid multimerization and low plasmid copy number. We conclude that mobilizable pBR322-derived plasmids can be used as shuttle vectors to transfer cloned genes to L. pneumophila, a feature that can be exploited for the purposes of mutagenesis or genetic complementation.

Physical and genetic mapping of the Salmonella dublin virulence plasmid pSDL2. Relationship to plasmids from other Salmonella strains

Plasmids of approximately 80 kb in size are found in nearly all clinical isolates of Salmonella dublin and are believed to be essential for virulence. We have shown previously that the 80-kb plasmid pSDL2 is required for the S. dublin Lane strain to establish a lethal systemic infection in BALB/c mice after oral or intraperitoneal inoculation. We now present a physical and genetic characterization of pSDL2. We have established a complete restriction endonuclease cleavage map of pSDL2 for five enzymes: Xba I, Bam HI, Xho I, Sal I, and Hind III. The region specifying autonomous replication has been localized to a 10.5-kb region of the Sal I A fragment by subcloning on the vector pBR322. Using transposon insertion mutagenesis with Tn5-oriT, a region encoding the virulence phenotype has been mapped within a 6.4-kb portion of the Sal I B fragment. Deletions generated by partial Eco RI restriction digestion demonstrate that at least 50 kb of the plasmid DNA are not required for replication or virulence functions, confirming the map location of these phenotypes. Plasmids of different sizes and restriction patterns were found in mouse virulent strains of S. dublin Vi+, S. enteritidis, and S. choleraesuis. By Southern hybridization, these putative virulence plasmids share a common 4-kb Eco RI fragment with the virulence region of pSDL2, and the plasmids from S. dublin Vi+ and S. enteritidis were shown to express mouse virulence comparable to pSDL2.

Identification of regions on a 230-kilobase plasmid from enteroinvasive Escherichia coli that are required for entry into HEp-2 cells

Certain strains of Escherichia coli can cause an invasive diarrheal disease in humans which clinically resembles shigellosis. These strains share with Shigella species the ability to enter and replicate within colonic epithelial cells and the ability to bind Congo red dye in vitro when grown at 37 degrees C. Like shigellae, they contain a large plasmid essential for virulence. A 230-kilobase (kb) plasmid from enteroinvasive E. coli was genetically marked with a transposon and mobilized into an E. coli K-12 background. This plasmid conferred upon E. coli K-12 the ability to enter and multiply within cultured epithelial cells, as well as the ability to bind Congo red. Expression of these phenotypes required growth at 37 degrees C. Transposon mutagenesis was used to identify regions on the 230-kb plasmid required for virulence. All transposon insertions which resulted in loss of the ability to enter epithelial cells, as well as the ability to bind Congo red dye, were mapped to a single 25-kb BamHI fragment. Subclones from this 25-kb region were tested for the ability to complement invasion in noninvasive derivatives. A subclone containing about 8 kb of the left end of the 25-kb BamHI fragment was capable of complementing noninvasive mutants with Tn5 insertions in this region and restored to these noninvasive mutants the ability to enter epithelial cells.

Genetic and physical analyses of group E exo- mutants of Rhizobium meliloti

Mutants of Rhizobium meliloti which are deficient in exopolysaccharide synthesis have been classified into six different genetic groups (A through F) (J. A. Leigh, E. R. Signer, and G. C. Walker, Proc. Natl. Acad. Sci. USA 82:6231-6235, 1985). Using physical and genetic techniques, we have demonstrated that the group E Exo- mutants carry deletions in the exoA-exoB region of the megaplasmid pRmeSU47b. We have constructed strains carrying defined deletions which remove up to 200 kilobases of pRmeSU47b, including the exoA-exoB region. These derivatives have the same phenotypes as do the group E mutants.

Gene transfer from Escherichia coli to Campylobacter species: development of shuttle vectors for genetic analysis of Campylobacter jejuni

A shuttle cloning vector (pIL550) has been constructed which can be mobilized from Escherichia coli to Campylobacter jejuni, Campylobacter coli, and Campylobacter fetus by complementation with the transfer functions of an IncP plasmid in trans, with a frequency of 10(-4) transconjugants per donor. We also present evidence for a DNA modification system in C. jejuni.

Construction and characterization of Bordetella pertussis toxin mutants

Pertussis toxin is one of the major virulence determinants produced by Bordetella pertussis. The DNA encoding the structural genes for pertussis toxin was cloned in Escherichia coli, and pertussis toxin subunit S4 was expressed under the control of the tac promoter. Mutations were introduced into the cloned toxin genes, and a conjugative shuttle vector system was devised for delivering the mutations from E. coli back into B. pertussis. The mutations were introduced by allelic exchange into the chromosome of B. pertussis resulting in a series of B. pertussis strains which were isogenic except at the loci encoding the structural genes for pertussis toxin. These B. pertussis strains were utilized to study the biogenesis of pertussis toxin. Polar mutations in the S1 gene led to a lack of detectable S2 or S4 subunits in whole-cell lysates, suggesting a polycistronic arrangement for these genes. Mutations in the S5 subunit gene resulted in a truncated S1 subunit, while mutations in the S4 gene resulted in a lack of detectable S2 subunit, suggesting that physical relationships among the toxin subunits are directly reflected in the stable biogenesis of the subunits.

Genetic manipulations in Rhizobium meliloti utilizing two new transposon Tn5 derivatives

Two derivatives of the prokaryotic transposon Tn5 were constructed in vitro. In Tn5-233, the central area of Tn5, which carries resistance to kanamycin/neomycin, bleomycin and streptomycin, is replaced by a fragment carrying resistance to the aminocyclitol antibiotics gentamycin/kanamycin and streptomycin/spectinomycin. In Tn5-235, the Escherichia coli beta-galactosidase gene is inserted within the streptomycin resistance gene of Tn5, and constitutively expressed from a Tn5 promoter. Both constructs transpose with about the same frequency as Tn5 in Escherichia coli and Rhizobium meliloti. When a Tn5-derivative is introduced into an R. meliloti strain which already contains a different Tn5-derivative, in situ transposon replacement is obtained at high frequency, presumably by a pair of crossovers between the IS50 sequences at the ends of the incoming and resident transposons. In this way we converted a previously isolated recA::Tn5 mutant into the corresponding recA::Tn5-233 strain, which can now be used as a genetic background in the study of complementation of other Tn5-induced mutations. We also replaced the drug markers of several Tn5-induced exo mutants, which we were then able to map relative to each other by transduction with phage phi M12. In a strain carrying Tn5-235 located near Tn5-233, we were able to isolate deletions of the intervening markers, presumably resulting from general recombination between the two transposons, by screening for loss of the Lac+ phenotype. Unlike Tn5 itself, resident Tn5-233 does not appear to suppress transposition of another incoming Tn5-derivative.

Second symbiotic megaplasmid in Rhizobium meliloti carrying exopolysaccharide and thiamine synthesis genes

Using physical and genetic data, we have demonstrated that Rhizobium meliloti SU47 has a symbiotic megaplasmid, pRmeSU47b, in addition to the previously described nod-nif megaplasmid pRmeSU47a. This plasmid includes four loci involved in exopolysaccharide (exo) synthesis as well as two loci involved in thiamine biosynthesis. Mutations at the exo loci have previously been shown to result in the formation of nodules which lack infection threads (Inf-) and fail to fix nitrogen (Fix-). Thus, both megaplasmids contain genes involved in the formation of nitrogen-fixing root nodules. Mutations at two other exo loci were not located on either megaplasmid. To mobilize the megaplasmids, the oriT of plasmid RK2 was inserted into them. On alfalfa, Agrobacterium tumefaciens strains containing pRmeSU47a induced marked root hair curling with no infection threads and Fix- nodules, as reported by others. This plant phenotype was not observed to change with A. tumefaciens strains containing both pRmeSU47a and pRmeSU47b megaplasmids, and strains containing pRmeSU47b alone failed to curl root hairs or form nodules.

The construction of a cloning vector designed for gene replacement in Bordetella pertussis

We report here the construction of a plasmid cloning vector, pRTP1, designed to facilitate exchange of cloned and chromosomal alleles of the human bacterial pathogen Bordetella pertussis. pRTP1 provides the ability to successively select two homologous recombination events within the cloned sequences. The first is by selection for maintenance of the ampicillin-resistance gene on the plasmid which is unable to replicate autonomously after transfer via conjugation. The second selection, via streptomycin (Sm) selection, is against the maintenance of vector sequences which contain a gene encoding the Sm-sensitive allele of the gene for ribosomal protein S12 thus rendering an otherwise Sm-resistant strain Sm-sensitive. We demonstrate the use of this vector to introduce an unmarked mutation, constructed in vitro, into the chromosomal locus encoding pertussis toxin.

Plasmid-mediated virulence in Salmonella dublin demonstrated by use of a Tn5-oriT construct

Salmonella dublin, a serotype which causes invasive disease in cattle and humans, carries a characteristic 80-kilobase plasmid (pSDL2). We were able to cure the plasmid from a strain of S. dublin. The cured strain was avirulent for mice by either the oral or intraperitoneal route of infection. A derivative of Tn5 which contains the transfer origin of the broad-host-range plasmid RK2 (Tn5-oriT) was transposed onto pSDL2, allowing mobilization of the plasmid by an RK2 helper plasmid. Reintroduction of the pSDL2 derivative plasmid into the cured strain restored virulence, demonstrating that the plasmid is necessary for virulence. These studies also demonstrate the usefulness of the Tn5-oriT construct for genetic manipulations.

Replication control in promiscuous plasmid RK2: kil and kor functions affect expression of the essential replication gene trfA

We previously reported that broad-host-range plasmid RK2 encodes multiple host-lethal kil determinants (kilA, kilB1, kilB2, and kilC) which are controlled by RK2-specified kor functions (korA, korB, and korC). Here we show that kil and kor determinants have significant effects on RK2 replication control. First, korA and korB inhibit the replication of certain RK2 derivatives, unless plasmid replication is made independent of the essential RK2 gene trfA. Second, kilB1 exerts a strong effect on this interaction. If the target plasmid is defective in kilB1, sensitivity to korA and korB is enhanced at least 100-fold. Thus, korA and korB act negatively on RK2 replication, whereas kilB1 acts in a positive manner to counteract this effect. A mutant RK2 derivative, resistant to korA and korB, was found to have fused a new promoter to trfA, indicating that the targets for korA and korB are at the 5' end of the trfA gene. We constructed a trfA-lacZ fusion and found that synthesis of beta-galactosidase is inhibited by korA and korB. Thus korA, korB, and kilB1 influence RK2 replication by regulating trfA expression. We conclude that the network of kil and kor determinants is part of a replication control system for RK2.