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

Plasmid pEC156, a Naturally Occurring Escherichia coli Genetic Element That Carries Genes of the EcoVIII Restriction-Modification System, Is Mobilizable among Enterobacteria

Type II restriction-modification systems are ubiquitous in prokaryotes. Some of them are present in naturally occurring plasmids, which may facilitate the spread of these systems in bacterial populations by horizontal gene transfer. However, little is known about the routes of their dissemination. As a model to study this, we have chosen an Escherichia coli natural plasmid pEC156 that carries the EcoVIII restriction modification system. The presence of this system as well as the cis-acting cer site involved in resolution of plasmid multimers determines the stable maintenance of pEC156 not only in Escherichia coli but also in other enterobacteria. We have shown that due to the presence of oriT-type F and oriT-type R64 loci it is possible to mobilize pEC156 by conjugative plasmids (F and R64, respectively). The highest mobilization frequency was observed when pEC156-derivatives were transferred between Escherichia coli strains, Enterobacter cloacae and Citrobacter freundii representing coliform bacteria. We found that a pEC156-derivative with a functional EcoVIII restriction-modification system was mobilized in enterobacteria at a frequency lower than a plasmid lacking this system. In addition, we found that bacteria that possess the EcoVIII restriction-modification system can efficiently release plasmid content to the environment. We have shown that E. coli cells can be naturally transformed with pEC156-derivatives, however, with low efficiency. The transformation protocol employed neither involved chemical agents (e.g. CaCl₂) nor temperature shift which could induce plasmid DNA uptake.

Identification and characterization of novel ColE1-type, high-copy number plasmid mutants in Legionella pneumophila

ColE1-type plasmids are commonly used in bacterial genetics research, and replication of these plasmids is regulated by interaction of RNA I and RNA II. Although these plasmids are narrow-host-range, they can be maintained in Legionella pneumophila under antibiotic selection, with low-copy number and instability. Here, we have described the isolation of two novel spontaneous mutants of pBC(gfp)Pmip, pBG307 and pBG309, which are able to mark the L. pneumophila with strong green fluorescence when exposed to visible light. One of the mutants, pBG307, has a single CG-->TA mutation in RNA II promoter located 2-bases upstream the - 10 region. Another one, pBG309, has the same mutation, as well as an additional CG-->AT mutation in the 76th nucleotide of RNA I, or in the 6th nucleotide of RNA II. A plasmid with the single mutation in RNA I, pBG308, was also constructed. Characterization of these plasmids carrying the enhanced green fluorescent protein (gfpmut2) gene revealed that the green fluorescence intensities of these plasmids were 2- to 30-fold higher than that of the wild type and both of the mutations contribute to increase the plasmid copy number and/or plasmid stability. The mutation located in RNA II promoter played a more dominant role in elevating the copy number, compared to the mutation in RNA I. We also tested the mutant plasmids for replication in Escherichia coli, and found that their copy number and stability were dramatically decreased, except pBG307. Our data suggest that these plasmids might be useful and convenient in genetic studies in L. pneumophila.

A two-component regulator induces the transmission phenotype of stationary-phase Legionella pneumophila

Pathogenic Legionella pneumophila evolved as a parasite of aquatic amoebae. To persist in the environment, the microbe must be proficient at both replication and transmission. In laboratory cultures, as nutrients become scarce a stringent response-like pathway coordinates exit from the exponential growth phase with induction of traits correlated with virulence, including motility. A screen for mutants that express the flagellin gene poorly identified five activators of virulence: LetA/LetS, a two-component regulator homologous to GacA/GacS of Pseudomonas and SirA/BarA of Salmonella; the stationary-phase sigma factor RpoS; the flagellar sigma factor FliA; and a new locus, letE. Unlike wild type, post-exponential-phase letA and letS mutants were not motile, cytotoxic, sodium sensitive or proficient at infecting macrophages. L. pneumophila also required fliA to become motile, cytotoxic and to infect macrophages efficiently and letE to express sodium sensitivity and maximal motility and cytotoxicity. When induced to express RelA, all of the strains exited the exponential phase, but only wild type converted to the fully virulent form. In contrast, intracellular replication was independent of letA, letS, letE or fliA. Together, the data indicate that, as the nutrient supply wanes, ppGpp triggers a regulatory cascade mediated by LetA/ LetS, RpoS, FliA and letE that coordinates differentiation of replicating L. pneumophila to a transmissible form.

Role of the 25 kDa major outer membrane protein of Legionella pneumophila in attachment to U-937 cells and its potential as a virulence factor for chick embryos

The gene encoding the 25 kDa major outer membrane protein (MOMP) of Legionella pneumophila was transformed into Escherichia coli JM 83 and the resultant E. coli LP 116 clone expressed the Legionella-MOMP. Compared with the parent E. coli strain, the clone showed a fivefold increase in opsonin-independent binding to U-937 cells. Furthermore, this gene was incorporated by electroporation into a low virulence derivative of Leg. pneumophila which showed reduced expression of the MOMP but enhanced expression of a 31 kDa protein in the OMP profile. After electroporation, the attenuated strain showed an increased expression of the MOMP while the 31 kDa protein was eliminated and virulence for the chick embryo was re-established. The use of a monoclonal antibody specific for the MOMP abolished virulence and adherence. These studies suggest that the 25 kDa MOMP of Leg. pneumophila serves as an adhesive molecule for host cells and that this protein plays a major role in the virulence of the organism for the chick embryo.

In situ detection of Escherichia coli cells containing ColE1-related plasmids by hybridization to regulatory RNA II

A method is described for the in situ detection of individual whole fixed cells of Escherichia coli containing ColE1-related plasmids. It makes use of fluorescence in situ hybridization (FISH) and the regulatory RNA II as a target molecule for both, Cy3- and HRP-labeled olinucleotide probes. Various methods for signal amplification were compared. Probes targeting the regulatory RNA I did not result in the in situ detection of plasmid-bearing cells.

The Legionella pneumophila hel locus encodes intracellularly induced homologs of heavy-metal ion transporters of Alcaligenes spp

We continued characterization of the Legionella pneumophila hel locus. Mutagenesis and DNA sequencing identified three genes similar to the czc and cnr loci of Alcaligenes eutrophus and the ncc locus of Alcaligenes xylosoxidans. On the basis of their similarity to these loci, we designated the L. pneumophila genes helC, helB, and helA. Mutations in the hel genes led to reduced cytopathicity towards U937 cells, although the mutant strains did not appear defective in other assays of virulence. Transcription of the hel locus was induced by the intracellular environment but was not induced by any of a variety of in vitro stress conditions. The function of the hel gene products remains to be determined.

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.

Effects of an isogenic Zn-metalloprotease-deficient mutant of Legionella pneumophila in a guinea-pig pneumonia model

To determine the effects, if any, of the Zn-metalloprotease on virulence of Legionella pneumophila infection, an isogenic mutant deficient in protease (encoded by the proA gene) was tested in an Acanthamoeba cell model, in guinea-pig macrophages, and in a guinea-pig pneumonia model. The cloned proA gene was completely inactivated by insertion of a kanamycin-resistance cassette into the protease gene of L. pneumophila AA100. This mutated gene was then introduced into the L. pneumophila chromosome by allelic exchange to form the isogenic ProA- mutant AA200. AA200 showed no difference in its ability to enter, survive, or grow in Acanthamoeba and explanted guinea-pig macrophages; neither light nor electron microscopy revealed morphological differences in the eukaryotic cells infected with the protease mutant or the wild-type strains. The proA gene was found to be expressed in L. pneumophila during intracellular growth in amoebae by measuring the light produced from a truncated luxC gene fusion with the proA promoter. Virulence of the protease mutant was attenuated when tested in a guinea-pig model of infection employing the intratracheal inoculation method. AA200 was slower to cause death, grew to lower numbers in the lungs, resulted in less necrotic debris and a larger macrophage infiltrate, and was more likely to be found in association with macrophage vacuoles than the parent strain. Although deletion of the protease was not sufficient to completely abolish virulence in a guinea-pig model, the mutation caused a delay in the lethal effects of L. pneumophila and attenuated the infection.

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.

PhoA gene fusions in Legionella pneumophila generated in vivo using a new transposon, MudphoA

To enable effective use of phoA gene fusions in Legionella pneumophila, we constructed MudphoA, a derivative of the mini-Mu phage Mu dII4041, which is capable of generating gene fusions to the Escherichia coli alkaline phosphatase gene (EC 3.1.3.1). Although an existing fusion-generating transposon, TnphoA, has been a useful tool for studying secreted proteins in other bacteria, this transposon and other Tn5 derivatives transpose inefficiently in Legionella pneumophila, necessitating the construction of a more effective vector for use in this pathogen. Using MudphoA we generated fusions to an E. coli gene encoding a periplasmic protein and to an L. pneumophila gene encoding an outer membrane protein; both sets of fusions resulted in alkaline phosphatase activity. We have begun to use MudphoA to mutate secreted proteins of L. pneumophila specifically, since this subset of bacterial proteins is most likely to be involved in host-bacterial interactions. This modified transposon may be useful for studies of other bacteria that support transposition of Mu, but not Tn5, derivatives.

Transposon mutagenesis in Legionella pneumophila. I.--Persistence of suicide and broad host-range plasmids

Two of three highly virulent strains of Legionella pneumophila could act as recipients at high frequencies in conjugation experiments with Escherichia coli donor strains carrying broad host-range plasmids belonging to incompatibility groups N, P and W. All broad host-range and most transposon-delivery plasmids persisted within transconjugants with high stability. Only one (pSUP1021) of several vehicles designed for the delivery of transposons into the chromosome of Gram-negative bacteria was found to yield transposon mutants of Legionella at a detectable frequency.

Replication of plasmids in gram-negative bacteria

Replication of plasmid deoxyribonucleic acid (DNA) is dependent on three stages: initiation, elongation, and termination. The first stage, initiation, depends on plasmid-encoded properties such as the replication origin and, in most cases, the replication initiation protein (Rep protein). In recent years the understanding of initiation and regulation of plasmid replication in Escherichia coli has increased considerably, but it is only for the ColE1-type plasmids that significant biochemical data about the initial priming reaction of DNA synthesis exist. Detailed models have been developed for the initiation and regulation of ColE1 replication. For other plasmids, such as pSC101, some hypotheses for priming mechanisms and replication initiation are presented. These hypotheses are based on experimental evidence and speculative comparisons with other systems, e.g., the chromosomal origin of E. coli. In most cases, knowledge concerning plasmid replication is limited to regulation mechanisms. These mechanisms coordinate plasmid replication to the host cell cycle, and they also seem to determine the host range of a plasmid. Most plasmids studied exhibit a narrow host range, limited to E. coli and related bacteria. In contrast, some others, such as the IncP plasmid RK2 and the IncQ plasmid RSF1010, are able to replicate in nearly all gram-negative bacteria. This broad host range may depend on the correct expression of the essential rep genes, which may be mediated by a complex regulatory mechanism (RK2) or by the use of different promoters (RSF1010). Alternatively or additionally, owing to the structure of their origin and/or to different forms of their replication initiation proteins, broad-host-range plasmids may adapt better to the host enzymes that participate in initiation. Furthermore, a broad host range can result when replication initiation is independent of host proteins, as is found in the priming reaction of RSF1010.

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.